OP: Great Escapes is not mentioned on these forums so there must be a reason.
Passport America has been around a long time and many RVers use it. It gives 1/2 off on campsites. It sounds like you'll be a very casual traveler after retirement and that's great. Passport America will work for you. I'd highly recommend it. A couple stays and you'll pay for the membership. It's not a membership that you'll be paying thousands for it.
So, along with Passport America, definitely get the national Senior Pass if you'll be 62+. It's $10 for a lifetime and will let you into all national parks, national monument with no entry fee. It will also give you 1/2 off on a standard site - no hookups. If a site has electric, it will give you 1/2 on the standard rate and you'll pay the electric part - not much. It's good at national parks, national monuments, forest service campgrounds, Corp of Engineer campgrounds, National Wildlife Refuges and other.