Hi Shellie! Welcome to IRV2! It's great to have you guys join us!
I wouldn't worry about the trailer and the ATV. It's true that many parks won't let you ride the ATV in the park, but I think most would let you park the trailer somewhere in the park and take the ATV outside the park to ride.
As for the length, you will find that it's mostly state parks that have the short spaces. Most private parks have plenty of spaces 40 to 60 feet length.
Good luck, happy trails, and God bless!
Joe & Annette
Sometimes I sits and thinks, sometimes I just sits.....
2002 Monaco Windsor 40PBT, 2013 Honda CRV AWD