Over the last 5+ years we've averaged $10 to $15 a night usually with water and electric and often with full hook ups though we prefer campgrounds. Avoid KOA's like the plague. They will always be the most expensive, full of kids and usually alongside a busy highway and or rail line.
We have never spent the night at Walmart, Cracker Barrel or any rest stops. Get a Passport America membership, it will pay for itself the second time you use it(typically a nice site with hook ups for$15-$20 (Sunday through Thursday but many parks honor it 7 days a week).
If you are 62 or older a Golden America (Geezer Pass), gets you half price at all national forrest CG's and free admission to the national parks and often a discount at State Parks. The Good Sam camping discount can help a little but 10% off of the rack rate is not much of a discount.
Stay with family/friends on their property every chance you get, might not be hook ups but a free night with friends and relatives is usually a lot of fun (though pretty tough to do with a large rig).
We just finished a 61 day/60 night cross country trip and spent less than $600 on CG's and RV parks. Works for us (24' Class C, see sig).
:B