The most restricted road I'm aware of is Going-to-the-sun Road in Glacier National Park.
This is very unusual, but vehicles are limited to 21 feet in length, 10 feet in height, and 8 feet in with including mirrors. Technically that rules out even most camper vans.
Other than this road which I wasn't able to cross because it wasn't fully open yet, I've only been limited at National Parks a couple of times for driving RVs up to 30 feet.
When traveling in our 20-foot van I do feel a greater sense of freedom. In one NP there was an area that didn't allow motorhomes, but there were quite a few van-size campers no longer than pickup trucks that were allowed to enter.