As S. Todd says, we get a 5¢ per gallon discount (off the cash price, and you can use a credit card) at Flying J and Pilot with our Good Sam Life Membership (not sure if it is the same with an annual membership). Most of the Flying Js have RV lanes, so much easier to maneuver than the regular lanes and you aren't mixing it up in the big truck lanes. Pilot owns Flying J, so you get the same discount there, but only the newer Pilot stations seem to have the RV fuel lanes. Nice thing about the RV lanes: no overhead structure, so you don't have concerns about height restrictions. Usually better maneuvering room, as well.
Love's has a loyalty card, but I haven't applied for it, so I can't say what or how much off they offer. I could be off, but I think they give you the cash price when using a credit card with their loyalty card, but it depends on how many gallons per month you purchase. Really designed more for truckers.
We use the Gas Buddy app to plan our fuel stops when traveling - it has been mostly accurate and you can get an idea of what the fuel prices are ahead. If your tow vehicle has the range, you can by-pass states with the highest fuel taxes by planning ahead; or, at least find the best price in an area.