Went across the country from the coast of NC to Olympic National Park and back in my Miramar 34.3 towing a Jeep with a bike rack on back which gave me almost 60' in length. Fueling up was a challenge we didn't relish.
I will have to echo what everyone else said here about the Flying J locations and the Good Sam's discount. Just make sure you have your Good Sam's card because you can't manually enter the Good Sam's number at the pump to get your Good Sam's discount- although you can manually enter a Pilot/Flying J account number and get the lower discount.
What I haven't seen mentioned and what we found to be MOST helpful and what I highly recommend is a book (they also have an app) called
The Next Exit. The great thing about this is that is lists for each state and for each interstate in that state the RV Friendly amenities for every exit- no matter if it were a Pilot/Flying J or not.
When we were on 1/4 tank my wife would get the book out and look for a RV friendly gas station along our route. it doesn't get specific enough to tell you if a location actually has separate bays for RV with gasoline but it never failed us on our 7,600 mile trip.
I also like the fact that you can get a book for those of us (like my wife) who aren't so savvy with apps.