There are several ways to keep rats and other rodents out of an RV's underbelly. One is to use a rodent-proof sealant on all openings leading into the area. This can be done during construction of the RV, or retrofitted to an existing one. Another way is to install a screen over any openings leading into the underbelly, such as vents or access panels. Finally, keeping the area clean and free of food sources will help deter rodents from making their home there.