Frequent Fueler Card at the Flying J, with their free showers that accumulate on the card, pretty much take care any cash discounts. I fill up at the RV or the car diesel pumps and pay with my debit card. Every couple of fillups I get a free shower and a free drink.
I can use my Comdata card and get the cash price at the big truck pumps at other places, but the Comdata transaction fee is more than the cash discount, and I can't get a free shower regardless at the TA, Pilot, others, because my fuel tank isn't big enough. Apparently, those truck stops think that if you have a small fuel tank, you don't stink.
Slow and steady, even in expediting, wins the race - Aesop