So, I'm sure you will actually get better answers from other people but I have some questions.
1. How long do you have?
2. How far south into the United States do you want to drive?

I would drive down the pacific coast, turn inward at Nevada, see the Grand Canyon, Moab, into Wyoming to see Yellowstone, across South Dakota, for the Black Hills, and Mt. Rushmore, Drive to Chicago, then onto Niagara Falls. That's a hell of a lot of driving, again how much time will help people offer suggestions, and I would also do a search on the boards for places you might want to go.

and I have no idea what a tent trailer is, so can't help you there

Thanks williamscb, we can be away for up to 7 weeks but don't think we will last that long altogether in comfined spaces. A tent trailer is a camper trailer (which is what we call it in Australia(, Basically a trailer, that pops up like a tent but is like a caravan with beds, kitchen etc. So we will need a tent/caravan site at places like RV parks etc.

We don;t want to go to South at all as going through the US is on our return to BC Canada where we will already have visited Canadian provinces. So thinking sbout driving through States/places like Boston, New York State, Minneapolis, Minnesota, Wisonsin, Montana, north Dakota (not in that order)
Spokane and then onto Vernon BC.

You will find the scenery/good sites and sights on this trip in New York State, Vermont, New Hampshire, Maine (no need to go above Brunswick), Cape Ann, Boston, the Berkshire Hills of Western Massachusetts, Chicago, southern Wisconsin, the Minneapolis area, Montana, and Idaho (oh, wow!). I will suggest specifics if you get farther with this plan.

I am of two minds about the tent trailer. I think camping trips and trips to cover ground are two different things. It takes a long time to get to a campground (they are not usually on a main road), set up, break camp, etc. Fine, if camping is what you want to do. Not so hot if you want to get from Niagara Falls to Burlington, VT in one day (and you do). I have lots of friends who saw the country by tent trailer as children, but they did it over a period of years, not in one fell swoop: this year, Calgary, next year New England. Look into the points systems for a few of the major hotel chains. Sometimes you can get a free night after relatively few paid nights.

There are some areas that may be tough. Boston and Chicago will be expensive. North Dakota has both bad roads and an oil boom. They are not unrelated, since the trucks are tearing up the roads. It will be expensive to stay because oil workers are crowding motels and cafes. There isn't a lot to see in eastern Montana and most of North Dakota except sky. Think "Nullarbor Plain" with cows. South Dakota has the Black Hills/Rapid City area.

We made a lot of long distance car trips when we were kids, and my parents' sanity was saved by (1) motels with pools and (2) frisbees. You can throw a frisbee anywhere, and twenty minutes of vigorous activity will calm your teenagers a lot (and can be enjoyed by the little one).

4 kids and 2 adults in a popup camper is a lot of folks in a small space. You may want to consider buying some pup tents or another pop up tent for the older boys to sleep in outside the big tent/camper.

Check out KOAs. In my opinion, they will be your best bets for campgrounds. Get to the campground early enough in the day for the kids to enjoy the pool - and tossing the frisbee. There's usually a game room and wifi available too. My kids enjoyed pancake breakfasts at their favorite KOA in Durango, CO.

For cross-country trips, we liked using the KOA Kamping Kabins instead of setting up camp every night. But you have too many people for the KKs. They do have Kamping Lodges, but I'm not sure that will be less expensive than staying in a motel room.

Have fun with this adventure! I love cross-country trips!

We are family with 3 children, We will go to Canada and EEUU from Barcelona (Spain) and we want to rent a motorhome too.
My question is: Is a safe for a family this kind of trip?

Renting a motorhome is safe but VERY expensive. Frankly, it will cost less to stay in budget motels and rent a car. Also, renting a motorhome for a one-way trip can be very difficult - it is often not allowed and if it is the drop off charge can be very high.

Finally, having a motorhome will prevent you from seeing major cities - which don;t have anyplace to put one. You need to leave it on the outskirts and then pay for public transit every day to get to and from what you want to see. For instance, the closest place you can leave a motorhome near NYC is in New Jersey - and the cost is about $90 per night. Plus the cost of bus or train to get in and out.

Motorhomes make sense if you are going to be in areas out west that have lots of places to put them - which may not be far from what you want to see. (But I would still do a car and budget motels.)

ah, so I read it backwards, you are going east to west.

then i'd do Maine, Massachusetts, Cape Cod, pop into NYC, Pittsburgh, chicago, south dakota, montana/ wyoming and back.

I suggest if you want to camp, start looking for campgrounds, national park, and even state parks along your route, and figure out the camping aspect of it, and let that dictate where you go.

Thanks starrs, ackislander, williamscb13 and others. These tips re what is worth seeing, where to camp is very helpful. We now have a new question.. how safe ae we going to be in a pop up camper in a trailer park? My husand is wondering whether we might be safer in the walls of a motel (albeit squished and without cooking facilities..) that at the mercy of crazed people who can have easy access to us in the walls of a canvas tent. Not sure if we have watched too many news reports about crime in the US when in Aust, but is it a valid concern?

> that at the mercy of crazed people who can have easy access
> to us in the walls of a canvas tent.
> Not sure if we have watched too many news reports about crime
> in the US when in Aust, but is it a valid concern?

Plain and simple, you've been watching too much TV.
A tent camper in a family-oriented camping area is just as safe as a budget motel. I've checked into motels that had me checking out in fifteen minutes after noting the locks on the doors had already been broken off, but I've never felt unsafe in a campground. There ARE some of the latter that I wouldn't check into, even if I were in an armored car, but that's where sites like Woodalls.com can help. If you want more specific ideas on how to feel safe as you use campgrounds, feel free to ask.

Brilliant, thanks.. Will look at that link.

That impression of campgrounds is really out there. They are among the friendliest and safest places there are.
I agree with Starrs about KOAs.
We did a 6 week trip with our children and camped part of the time using Parks and KOAs. There are camping directories available.
What kind of vehicle do you have? Rather than the pop top, you might get good camping equipment and a car top carrier. You may need the carrier anyway for children's space in the car.
Do they have DVD players, etc.
You will really not be able to navigate large cities with a trailer camper.
Your return through the northern US is a little problematic--they are fine states, but there isn't a whole lot to do.
I do like the Badlands. Wyoming will offer you some good scenery.
The northeastern states are very interesting and beautiful.

Boston, New York State,

These are out of your return trip to BC. It will be a full day's drive to Boston, and then back to the longitude that is represented by Toronto or Niagara Falls. There is no way of avoiding long stretches through the Plains states, but you will have discovered that driving across Canada. Very little that is appealing between Chicago and the Rockies. Try to do it in two days.

From what I have seen of private campgrounds, I am not crazy about them, although they may have amenities that are not available in public campgrounds. We prefer staying in National Forest campgrounds or National or State Parks campgrounds. I think that they usually have more space between the camp sites.

Obvious stops on the way back: the Black Hills, Yellowstone, Glacier, Rocky Mountain NP, Dinosaur National Monument, Lake Tahoe, Crater Lake. Guidebooks might help you make a choice, using a map to rationalize the itinerary. A project for the 12 and 15 year-olds.

Browsing through these sets might give you some ideas:

http://www.flickr.com/photos/mksfca/collections/72157624514560099/

http://www.flickr.com/photos/mksfca/collections/72157624514508361/

I don't disagree about the comment about campgrounds, but if you are just travelling and want to stop, they are fine. If you are going to stay in the National park campground they also fill up early in the day. Some you can make advance reservations (we did at Grand Canyon for example).
For the teens, the commercial campgrounds also have game rooms--not a bad thing after a day of driving across long plains.
We drove from the east (NC) to the Pacific on our long family trip and one reason was to show our kids just how HUGE the midsection of our country is. They know, but they weren't thrilled!!

Oh you HAVE been watching too much TV! You will be completely safe in campgrounds. Nice, friendly people. MUCH better than a budget hotel, IMO.

Since you are doing so much moving, seriously consider staying at KOAs. You can make reservations ahead of time and take that worry away. The kids will enjoy the swimming pool - and game rooms, etc. - and you'll be able to spread out more.

Enjoy the trip. I loved camping with kids.