The shorter you are, the more campsites you will fit in.
A 20' rig can park in a 40' site but a 40' rig cannot park in a 20' site.
If you are not reserving in advance, your best bet is to try first come first serve campgrounds during high season where-ever you may be.
Having full-timed for 3 years in a 33' A, I can say that:
1. if I want to stay in a specific state/fed park and they have reservations, the earlier I make them, the better chance I will get a site I like that I can fit in.
2. There are parks that I love that I must make reservations way in advance or during the off season to get one of their few sites I will fit in.
3. There are parks that we used to go to with our 28' tt that we can no longer go to due to length restrictions.
4. There is almost always a private park in the area of where you want to be that can accommodate larger rigs.
Just an example from my search this morning - Fish Creek Campground has 173 sites. 23 of them will accommodate up to 35'. Arriving on June 9, 2017, there are 7 sites available. 2 of those are handicap sites, 2 are in the no generator section, and three are tight pull-thrus. There are private parks in the area and a first come first service park in the area, so you have other options. They may not be what you want, but there are options.