I use Open Roads as many Bus Conversion owners do, and get up to 40 cents off per gallon at many truck stops.
The Costco's and Sam's Club's around here in the midwest either don't have diesel, or if they do, have islands that are not roomy enough to handle large vehicles. Chains like Casey's or Circle K can have more room. If the price is good I don't mind waiting to fill with small slow nozzles, but the the lower volume stations have more opportunity for contaminated fuel clogging the filters.
I like filling with small nozzle's ,no foam, time to walk around and do checks and clean the windshields and walk the pets ,plus they are cleaner.I liked the older Flying J's you could get fuel,dump you tanks,fill with water,and buy propane if needed all at the RV island with out moving and there was no dump fees