Some places you get a better deal with CASH, others with a specific credit card (MUST BE THE PROPER CARD) Flying-J/Pilot for example. Have done it all ways.
You may wish to join Good Sam's and/or FMCA (Family motor Coach Assn) as some stations discount if you flash/scan one of their cards first. (Same Pilot/Flying-J) There may be other discount programs as well.
Another option: Some credit cards offer rebate programs.. Shop for those as well.. As much fuel as you will burn if you travel full time.. Might buy the odd dinner or 3.