An app I strongly recommend you get is GasBuddy. It will help you find the cheapest fuel in an area. The question is - do you want to go to that station? If you are not traveling in your RV, it might be worth the lower price. If with the RV, you have to consider access.
I've used grocery store chain loyalty programs, Pilot/Flying J card, Walmart card discounts, bulk club stores (Costco/Sams) and any other discount I can find for fuel. The search for cheap fuel is constant and there is not one answer. My limit is five extra miles driving for the truck while towing the trailer, 10 extra miles driving to reach a station with a cheaper price for the truck alone.
(Okay - there is one best answer - a friend retired from a management position with a major worldwide oil company which has retail stores in all 50 states. He gets a 30% discount rebated to him on his statement for fuel purchases at company stations with the company sponsored card. But very few of us have retired from that company.)
Grocery stores are usually regional. And just the two of us in the rig full-time and not having room for much storage of bulk purchases (size or weight) - we just don't spend that much money to add up often.
Walmart associated Murphy USA stations often are configured so that we cannot get the rig into the station to get fuel.
And we've traveled 20,000 miles in 2 1/2 years - so our access to grocery chain discounts is limited at times.
The Pilot/Flying J RV Plus Card gives us 6 cents per gallon off the pump price of gasoline - anywhere we have been. From New Mexico to Florida to Maine and back to Texas.
Many to most Flying J's have RV lanes which make it easy for me to get into and out of the station. A few do not. Very few Pilots have RV lanes (one which does is on I-37 just south of San Antonio). Some Pilot stations I can get into easily, others (the one in Tyler Texas or Arkadelphia, Arkansas) I cannot get into with my rig.
Sometimes the cheapest fuel is at a station that is too far from my travel route, along too many small streets, etc.
Sometimes convenience is more important than price.
I find Flying J and Pilot stops to be very good matches for RV's. We know what they carry in most stores. We can get what we need. There is almost always a restaurant or other hot food at the stations.
If I want to sit down and relax, I know there is parking available for the full 62 foot long RV rig. Sometimes not in the front.
I have no qualms about taking my truck/ trailer around back and putting them into a big rig spot. The only comment I've ever gotten - is a frequent - "Danged, I didn't realize how long your truck and trailer combination were."
I will do the same thing at Love's, Petro, Travel America or any other truck stop.
The P/FJ Card has no extra costs associated with it - other than having a Good Sam Membership - which pays for itself with a couple Camping World discounts.
I strongly recommend getting one.
Warning - get the P/FJ book of all their stations - and learn how to determine which are company locations and which are franchisee locations. Many franchisee locations DO NOT honor the card for gasoline discounts. Most do honor it for truck lane diesel fuel discounts.