The best discount card is the one of the several US annual or lifetime park passes. The Access Pass is best if you can qualify. No cost and lifetime, gives you free entrance to parks and 50% off camping in any federal campground.
Good Sams gives you a laughable discount at some fuel stops, but those places are so grossly overpriced that the discount is worthless. I think it is 7 cents a gal while the diesel was priced 50 cents a gallon above other stations. GS also gives a 10% discount on camping at one of their many parks with is more useful.
PPA Passport America gives a one third discount for one or two days with a lot of exceptions. But they have fewer parks and they are not often near major attractions. But it will pay for itself quickly if you use it. We stayed at 37 places this summer and I think we used a PPA park once.
Escapees gives I think a 15% discount at a very limited number of parks. We never seen one actually.
Boondocking gives a 100% discount, no card needed. But getting to the point where you can do it for 3 or 4 days has its own one time costs.
Mostly East of the Rockies the Corp of Engineer(COE or they called them USACE) CG are excellent parks and cheap especially if you have one of the federal passes. Most are water/electric tho. FHU is unusual. The Forest Service has a lot of parks but most don't have utilities tho some do. They are cheap.