We tow a 36-foot 5th, and I can talk specifically about the options we encountered on our last trip.
Jesse M. Honeyman SP, Florence, OR: Plenty of suitable sites.
Crater Lake NP: The only campground inside the park had virtually no sites we could fit into without a lot of difficulty. (Every site is an arced pull-thru with obstacles close by, and we find these to be a challenge for our 51-foot rig.)
Panther Flat NFS CG, Gasquet, CA: Plenty of suitable sites.
Lassen Volcanic NP, Manzanita Lake CG: Plenty of suitable sites.
So 75% of the SP, NP, NFS, etc. campgrounds, there was no problem. Bottom line is you will be restricted at times, but I wouldn't let that stop you from getting the length that fits your needs.
2015 Crossroads Rushmore Springfield
2015 Chevy Silverado 2500HD Duramax