RV groups you can join with a (usually) low membership fee and in return get discounts on RV parks, parts, and services.
Good Sam - Offers a multitude of services for RVers. Members get a discount at Good Sam parks (usually 10%) They also offer roadside assistance for your vehicle and RV and extended service plans. Low yearly membership fee.
Passport America - Offers a 50% discount on Passport America affiliated parks although I've heard that these discount can be seasonal. We worth it for the low membership fee.
Escapees RV Club - Offers a multitude of services for the RVer. Park discounts, mail forwarding service, roadside assistance, and more. Well known for their advocacy program protecting the legal rights of RVers.
Thousand Trails - A large network of RV resorts and campgrounds. Expensive but if you intend to use it a lot it could be worth your while. Memberships are also available for resale. Think timeshares for RV parks.
Are RV memberships worth it? - An article with more in depth info about memberships. (Heath & Alyssa Blog)