I have a Cedar Creek 34RL which is very close to 37' long and with my TV I am close to 55' in total length. We have camped in some Federal Parks mainly COE's parks that offer 50 amp full hook-up sites that are over 40' in length. But very few state parks; mainly those west of the Mississippi River. I search each state/Federal park for site size and to see if they offer 50 amp full hook-ups sites. A lot are just 30 amp electrical and water sites with no sewer or just 30 amp electrical only. We than use Google Earth to review the park to see if there are a lot of trees and to see what other campers have to say about the park before booking the site.
I can not see the value of camping in a state/federal park that could destroy your $60K+ 5th wheel; do to a lot of trees branches scraping the sides and tearing the rubber roof. Plus not offering full hook-up 50 amp site. So we tend to use private parks that caterer to the large 5th wheels and motor homes that trim back the tree limbs and so forth. Plus they offer full hook-up 50 amp sites, with cement level pads.
2016 34RL CC; 2008 Ram Mega Cab 2500HD, 6.7L, 68RFE 6 speed, 4X4, Smarty S67