Screen material and a glue gun are how I sealed most of the critical areas of my trailer, such as refrigerator inlet door and battery box. I also sealed the bottoms of my propane tank bays with screen material and metal screws. Commercial screens are available for the hot areas where typical fiberglass screen material won't work, like furnace outlets and water heater. If you still have your "mouse hole" for the shore power cord, do the conversion to the locking/removable cord.
If you're looking for a way to keep them away from the rest of the trailer....a sealed garage with a ray gun to zap them if they make it inside?
They're pretty tenacious. I keep them out of the critical areas, but pretty much let them have the run of the rest of the place. I'll go through and knock down their nests during the winter when they're not active. Smart, huh?