I have three main discounts that I use: Passport America, Good Sam, and KOA.
Passport America: It only cost $44 a year and the benefit is that you only pay about half of what the normal cost of the CG is. So in four or five nights of staying at a PA campground, you would get a return on your money. Assuming a normal rate of $40 a night.
Good Sam: It is only a 10% discount, but it does add up after awhile. Good Sam is $19 a year. It takes awhile for a return on your money.
KOA: It also is only a 10% discount, and you do have to be a "Valuecard" member for the discount. KOA Cost $24 a year, so at 10% it takes a while for the return. The biggest advantage to KOA is the ease at finding a site in a an area that you are going to be in. It can all be done on-line, and with good service. Also the majority of their CG's are very clean and nice, although I have been in a few that would not quite be up to our quality.
I have not traveled in the North Central part of the US yest, but we have traveled to the North East. KOA's can be found, but all-in-all the are on the pricey side. More so in our travels to the North East. Passport America becomes less available as you travel North East. I think that is because they are so seasonal for the camping time. A 50% discount for a CG that is only open 6 months or so is not a good money maker for them, so they tend to not have PA, or they become very restrictive with the days you can stay, and for how many days. You can see for yourself at Passport America