The question is, what warranty? If you are talking about an RV component such as a fridge or water heater, any shop that is authorized by the manufacturer of that component to service it can do warranty work. If you are talking about an RV manufacturer's warranty, it has to be a dealer for that brand, though the factory may authorize someone else if you ask nice. If it is a chassis part, the it has to be an authorized service center for that brand of chassis.
And if you are talking about an extended warranty, that's not warranty work at all. It's just insurance and any shop willing to do the paperwork and phone call needed can do the work.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL