I can only comment on Yellowstone.
You have two basic choices. First, keep in mind Yellowstone is a 100 mile by 100 mile square Park as the crow flies - in other words it's HUGE. Your first choice is staying outside of the park and driving all day long to get to the things you want to see in Yellowstone or your second choice is staying at centrally located Fishing Bridge in Yellowstone next to a large lake, because it's the only place that can support large rigs but it's cramped spacing.
Outside the park you'll have more space at your site, but if you are gone all day because everything is a longer drive then you won't be spending much time at the CG anyway. Expect to drive about 150 to 200 miles a day at about 45 mph slower driving if you want to go to all corners of Yellowstone which may not allow a lot of time at each point of interest.
Fishing Bridge being centrally located is a 75-100 mile trip to each corner of the Park (half the distance) and many less hours in driving time because the more you drive in Yellowstone the longer it takes because of stoppages, traffic and animal watching. I hope you are recognizing that I am highly recommending staying at Fishing Bridge even if it's crowded.
Two other considerations. I went to Yellowstone in late September to beat the summer crowds, which we did, but we ended up in an early winter storm. The daily temperature averaged 39 degrees even with little to no precipation. Fortunately, I had the foresight to pack parkas and ski pants when we left home even when it was 91 degrees outside at the time. The other thing is Fishing Bridge is the only campsite that allows for advance reservations. Look for the information on the internet and lock in a campsite. You will need to provide the length of the RV and the TV so they can fit you in.
NEW EDIT!!!!!!!!! Fishing Bridge will be closed when you get there. Besides Madison and Mammoth the link below shows other campgrounds that might still be open. Check the name of the CG and when it's closed. Then you will need to make sure your rig will fit.
http://www.yellowstonenationalpark.c...ng.htm#Camping - Yellowstone National Park