My experience is, it depends on the resort. Some are class A only, some allow class A and class C, some have age restrictions on the coach, some don't. I have a lot at Riverbend Motorcoach Resort near Ft Myers in Florida and a lot at ORA Pacific Shores in Newport, Oregon and you would be welcomed at both but neither accept 5th wheels. However, some of the Outdoor Resorts sites DO accept 5th wheels. This information is not always on the web site. If you find somewhere you like and they don't specifically say they exclude 5th wheels, I would still call ahead and check.
Mick