1. RVs are nothing like the auto industry. While a local dealer may work on it, you more than likely be plafed at the bottom of the list.
2.length limits is all park dependent. In NF parks at least in the far west most were built in the 50s and 60s. They were built for tents and very small RVs. Some have been upgraded, but not many. We used to camp host in state parks. Our favorite park was built in the late 60s. We started camping there in the mid 70s. Other than upgrading to 50A service in part of the park and new pavement the past few years, the park is exactly the same as when built. Meaning the sites do not handle todays super sized RVs.
Generally speaking if you want to visit state and NF parks, stick with the smalles RV your comfortable with.