Each state is a little different - Here in CO we stay in many of the improved state parks without any issues, but the National Parks are still a little primitive here. The smaller parks seem to have been designed around the 22-ft Dodge Class C (some of them are still camped there
).
As Rich said, you just have to ask around, call around, and experience first hand. The bigger handicap is the slide width, not the length. Our 2003 2955RL fit in more of the parks with trees, but because it only had slides on one side, not because it was 5-ft shorter. Keep a log for return visits and mark the sites you might like better next time out.