As mentioned above, National Parks are only part of the concern. There are way, way more campgrounds that are in National Forests or on BLM land. Many of them have even more limited access for larger rigs.
There is no magic limit. The bigger the rig, the more issues with finding suitable sites. It seems to me a great many RVers end up with very large rigs. I am sure the salesmen help with that. Personally I recommend looking at the smallest rig that will meet your needs. My wife and I, along with 2 cats, lived full time for a couple of years in a truck camper without slides. We had what we needed: 3 burner stove, furnace, hot water, toilet with shower, AC, solar panels, 2 bowl kitchen sink, and a full sized Queen foam mattress. How much more do you need?
