What campground association to join?

Good Sam is quite often good for a discount. For overnight stops we use Passport America a lot. Pays for itself over and over. Often the campgrounds are small mom & pops and occasionally a bit iffy for anything more than an overnight. That said we have also found a number of small family campgrounds that have been delightful.

When we cannot find a convenient PA campground the next stop (which I always check anyway) is RV Park Reviews

We have the Good Sam and KOA memberships which we have used all over the United States. Most have been good campgrounds, some average, and a few poor campgrounds.

GS and Escspees for us..

I have Good Sam for Camping World and fuel discounts, and the occasional campground discounts. We travel a month each summer and have not seen the need to purchase anything besides a national parks pass and Texas State Parks pass. RV Park Reviews and general internet searches give us the results we want for campgrounds; whether or not the campgrounds we choose provide a discount is an afterthought.

X2 on Passport America. Agree that some are so-so, strictly for overnights and some better...we stayed in three that were quality campgrounds in prime locations.
(So I always check RV park reviews.com to get the scoop).

More than paid for the membership with one use in the Florida Keys.

There are limitations the park sets such as one night only, five nights maximum, not in July and August etc. You can look at their website, click the states you want and see what's offered there, without joining first. That would tell you if it will be of use.

We also use Good Sam a lot.

We belong to Passport American and Good Sam. If I were you I would wait before joining any except Good Sam until you know your lifestyle KOAs are expensive and we never got our money out of it. You might find that you prefer to camp at National/State Parks instead of private parks and so an Eagle Pass/Senior Pass is a must and only 10 bucks

Passport america is great but check the catalog listings for locations and restrictions. two nights a year will pay for the membership. that would be my recommended first and maybe only organization to join.
Good Sam's discount is the same as AARP, AAA, etc.
bumpy

I don't use any instead choosing State parks, COE parks and better NFS sites and find them to be a bargain still.

X-2 with fla-gypsy.

You said in your post you were going to be looking for retirement land. I hope you might consider adding NE GA to the areas you might check out. There's lots of cgs in our area where you can comfortably stay while you look around. We are near where the Appalachian Trail starts. Lots of waterfalls. Hiking trails. Lakes. And so much more. Pleasant weather. Far enough away from the big city (Atlanta) but close enough for the airport if needed.

As for cg memberships. We have Good Sam & Passport America. They both have great apps for my Android smartphone. (I understand the apps are also available for iphones) I like using the apps because I can read & use it easier than the thick cg catalog books that I need a magnifying glass for!

Check out the different membership groups online & see which one will be the best fit for you - if any are.

Enjoy your visit to the Southeast! Beautiful area! Different beauty from Colorado but stunningly beautiful!

If you want to add NE GA to your areas to check, I'd be happy to give suggestions of local cgs.

Take care & safe travels!