I have a 36' DP A and we pull my wife's Kia on a dolly (as in my signature) and speaking East Coast only we have not had any length issues finding places to stay. This would include Ormand Beach State Park in Florida where the online reservation system said 30' maximum length. I found online pics that showed large 5th wheels stuffed in the sites and a couple of class A's. I took a chance, and though snug (told my DW one of my better parking jobs) our rig, car, and dolly fit nicely.
State Parks aren't going to be an issue for the most part because as you probably already know besides being cheaper, most SP campsites are huge compared to privately owned parks. I have yet to stay at National Parks, only their sister parks Army Corp of Engineers and again their sites are also large.
sherman12 and The Fabulous Miss Barb
2014 SportsCoach Cross Country 360DL DP
Former 2011 Georgetown 330TS
Kia Sportage on MasterTow Dolly