With our 38ft 5th wheel we stay on primary & secondary roads unless we know the road and possible obstacles. I always use the rule of thumb if a semi can use the road so can we.
Near most state and national parks you will find at least one if not many private RV parks. We choose to stay in private big rig friendly RV parks when visiting a national or state park area.
As far as towing the bigger 5th wheel with a properly setup tow vehicle, you'll feel confident after several trips. The most important thing to do is take the 5th wheel out to a big field or empty parking lot and practice, practice, practice... How does it react during a panic stop, quick lane change, backing...etc. Setup a tight parking space using cones and practice backing in to the spot.
For fuel, I usually stop at the large service stations or truck stops located near the interstate or I will go in to town and fuel up before hitching up and leaving from an RV park.