We're planning our RV trip, and from what I understood, there is quite a bit of overhead to park the RV for overnight stay, and therefore recommended to stay at same place for at least 2 nights.
My question is, during traveling from point A to point B, is it practical to stop for a day trip in point C, or should we plan the trip that there will be no day trips unless we stay the night at the spot?

Your question is unanswerable. Wouldn't most everything depend on which points A, B and C you are talking about?

Whether they are they are 'commute-able' or not.

RVs can stop for the night without hooking up to all utilities but you will have to watch battery and gray water amounts. (i.e. don't use the shower) While on battery power, you will not have full use of electricity such as microwave, if there is one but stove and frig should be running on propane. DH was able to stay in a state park without utilities for a weekend. Use the campground facilities instead of your RV bathroom. Hooking up the electric isn't hard, it's the sewage pipe. Time also depends on how much work it is to get backed in, level everything up, put out the awning, etc. If you have a level site and want to rough it a little, you can.

Sometimes we plan a route where we change campgrounds halfway thru the week and use the change day to visit something along the way. Also, when we are driving, it's great to find a quiet spot for lunch. We have everything we need right at hand.

So basically RV's allowed to enter state parks for day use only?

It depends on the State Park . . some will have overnite capabilities and some will not.

We traveled by RV for years when the kids were growing up. We could be set up in less than 5 mins . . un-setup in less than 10.

I would take care of the sewer connection while bride took care of the water and Electricity . . the kids would head to the pool or playground.

Yes, you can stop enroute with your RV, provided that there is enough room to park the RV. Many state parks let RVs come in for day use. The deciding factor is usually space. So the smaller the RV, the easier it will be to stop places enroute.

A really good forum for your RV questions is www.rv.net/forums

Great folks with answers to just about any RV related topic.