If you decide to fly and rent a car don't forget about the possibility of huge drop off charges if you don't return it to where it was rented.
Even paying for luggage, IF you want to camp, paying for one backpack (be sure of weight and measurements) of equipment could be economical--but be sure to check availability of a campground (even a KOA) for your time. They fill up in peak season. Make reservations if possible/necessary.
We did a long western trip with our children and camped about 1/4 of the time (6 week trip) and it was fun. There are whole books of commercial campgrounds==and can probably be searched on the internet.

I read a bit more about Southwest and it sounds like they are more likely to institute change fees than start charging everyone for luggage. They've spent so much money on branding themselves as the no-luggage fee airline that it would be a major negative to go back on that now...but they very well could.

I also have an American Express card with Delta that allows me to check one bag for free. If I travel with someone else and book their airfare on my card, they are also allowed to check one bag for free. I don't know if other airlines have similar programs but you may want to look into that.

If you have your own camping gear, start by seeing how much you can fit into a standard piece of luggage. I've been able to fit the basics (tent, sleeping pad/bag) into my luggage along with clothes for a week. You might want to consider buying some cheap gear out there to save space in your luggage - I remember finding a huge tent for under $50 in Utah once. I didn't buy it and I'm sure it wasn't good quality but it would've lasted for a couple weeks I'm sure.

Also, you can sometimes camp free in National Forests, or at least find a decent campground when a nearby park is full. Like Gretchen says, definitely make reservations and don't just show up. There are some first-come, first-served campgrounds in the parks but you'll have to be there early to try to snag a spot. It's much easier to just reserve something and not stress about it.

I went to RMNP last summer and had no problems finding a campsite. I had some days reserved, other days were open so I could move to a different part of the park if I wanted. But I had the luxury of time on my side as I was there for over a week and it didn't matter much if I had to wait for an hour or so to claim a site. With your tighter schedule, I would make reservations.

Oh and lastly, I just looked at Amtrak to Glacier from Pittsburgh. It's about 40 hours on the train with a layover in Chicago. Lowest cost I saw was $500 round trip per person. I don't know if you could get the cost down but the travel time would be a deal breaker for me. I would think you could fly to Glacier for about that much and you wouldn't spend 4 days getting there and back.

It's the "round trip" that's the problem. They are going multiple places. Trains are just too late and too long any time you are on any kind of "schedule" that means anything.

I think that "fly to Glacier" is not a good option. The closest airport is at Kalispell.
If you wanted to fly to a larger airport try flying to Spokane and take the Empire Builder eastbound at 1:30AM. You arrive in Whitefish before 8AM.
The last time I took the EB was westbound and we arrived in Spokane 30 minutes early.
If you buy Amtrak tickets more than 2 weeks in advance, the round trip from Pittsburgh to Whitefish comes out to $416pp.
If you wanted to go further on Amtrak, you can buy a 15 day USA Rail Pass for $439 per person which includes 8 "segments".
I have used several 30 day USA Rail Passes "12 segments".
Being retired I only fly when I have to be somewhere in a hurry. I consider what I see out the train window as part of the destination.
Remember that any night you spend sleeping on a train, is a night you are not spending over $100 on a hotel room.

"If you buy Amtrak tickets more than 2 weeks in advance, the round trip from Pittsburgh to Whitefish comes out to $416pp."

I'm not sure if you have some discount or something, because when I check right now for a trip in 3 weeks, Pittsburgh to Whitefish is $342 one-way, going back it's either $323 or $342.

The price of about $500 I mentioned was Pittsburgh to East Glacier for next June. That was the latest I could price it out.

I wouldn't recommend taking a 40 hour train ride to save on hotel costs, especially not when the OP is willing to camp for $20-25/night.

Actually I would rarely recommend taking Amtrak, I simply included it to cover all the bases since the OP had asked about it. There are times I would consider it. But for me personally, there's no way I'd recommend the train for this particular trip.

I'm certainly not against train traveling but I'm not sure how well I'd do being confined to the same space for 40 hours.. kinda scares me just to think about it. I'm not a good sleeper while traveling. I like to be awake and take it all in. I could end up killing myself if I tried to survive a 40 hour train trip. :)

We are currently trying to decide if 2 trips is the best way to go. I don't want to cut everything out but it WOULD make our lives easier if we did a Badlands/Yellowstone/Grand Tetons trip and a seperate Montana trip the following year. I would be perfectly content to take a vacation that solely focused on Montana, maybe stop off at a dude ranch. That would be heaven for me! I don't know about the other guy who I'm dragging along with me but hey I'm the one doing research here. That's how it works, right?

IMHO Amtrak is almost as much as flying AND using up valuable time even ASSUMING they are on time. I've seen Amtrak arrive here in Whitefish sometimes 6 hours late!

Think about Custer State park -- it really is nice and gets little attention. I wasnt expecting much and was so surprised to see how beautiful it was and much less crowded then the bigger parks. RIght near the Badlands so an easy combo.

Custer is definitely on the radar now. It sounds like a long stay isn't warranted in the Badlands so I think we could make time for Custer. I'm all about the beautiful views. :)