FMCA has just started a fuel discount program named TCS (not to be confused the TSD mentioned above).
https://fmca.com/rv-fuel-discounts
The reports I have seen elsewhere claim that TSD has deeper discounts and does not require a pre-pay account as TCS does, but I have no experience with either one myself.
Good Sam has a fuel discount program with Pilot/Flying_J but it's a much smaller amount/gallon than TSD or TCS.