Have you ever seen the insides of a wheel on a car after someone sprayed in Fit-A-Flat? What a mess. and most tires shops will charge you extra to remove all that goop before they can mount a new tire.
I'd stay away from this stuff -- because sooner or later those tires on the RV will need to be replaced from age or thread wear. And buying 6 new tires is going to be a major hit on the wallet. You don't want to add in several hundreds more to clean that sealant off the 6 rims as well.
Only way I know to prevent flats is to leave it parked -- but then, you're not RVing.
Ed & Bev Felker
Retired USAF Col and retired Nurse
Traveling with Lily & Bella ('Teddy Bear' breed)
2015 Vegas 24.1 (E-350)