Hmm, I don't think there is a relay. Just a pressure switch up in the front of the RV under the 'hood'. Screwed into the AC tubing. You can check your RV's wiring here:
Wiring Diagrams and that should lead you to that pressure switch. There's 2 of them in my system, both under the hood. (Diesel pusher). I have the same set of fans on the drivers side rear that you do. I think your pressure switch is bad.
Once you know which is which, you can just jumper across the two wire terminals and let the fans run (assuming you're driving where it's hot), won't hurt anything, meanwhile you can order the switch. Or probably find it at Napa. Usually it was the threads that were different so it's best to know which threads yours has.