I just saw that it is a trailer you are getting. That is big! Actually it is very big for just two people. There are some sites in Yellowstone that will work for you, but none with service since Fishing Bridge is out of service currently. There are many state parks here in Georgia that accommodate big rig motorhomes with sites 50 or more feet long and then space to park the tow truck.
At one time we had a 30 ft trailer and it was very adequate room for tow of us.
Buy what you are comfortable with, but like others said, there may be limited sites in some placee