Escapees, Good Sam and FMCA are RVing organizations that offer more than discounts at RV parks. I am a member of some of them because they have good magazines and other benefits.
You will only know if a club like KOA or Passport America will save you money if you do the math yourself based on where you stay. For example, we found that were staying in KOAs a lot one year because of the places we were going to and the KOA membership would more than pay for itself. Where do you travel and where do you like to stay? Then you can figure out if it is worth the cost.