What campground association to join?

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

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.

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.

GS and Escspees for us..

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.

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