For what purpose? Coachnet is an excellent roadside assistance program, Good Sam encompasses campground discounts, Camping World, rallies, and several magazines as well as selling roadside assistance and repair insurance, FMCA is for motorhomes and has an excellent magazine and numerous rallies. For support of the fulltime RVer, there is also Escapees. There is no one organization that offers everything. Many of us belong to several, or all, of the above.
Tell us exactly what you're looking for and we can give you a proper answer. Also, searching the forum for a subject will give you lots to read as all these topics have been discusses many times before.