Some places give a discount, others not. Yes check to see if you can get a credit card with a rebate to buy gas.
In Arizona, if you have a commercial truck, then you pay an extra 26 cents per gallon for truck stop fuel, for trucks over 26,000 GVWR. Motorhomes are exempt,, but would normally need to get fuel at the car pumps in order to get the reduced price. Other states might also have a fuel tax based only on truck GVWR, but I don't know.
It sure is nice to have dual 26 GPM fuel fillers when filling a 120 gallon tank. But even with 1 filler, it goes pretty fast at a truck stop. Car fillers are only around 8 gallons per minute, be patient, it will take 10 minutes or so. . .
Fred.