Greetings from Mikey at Just Answer!
Let me see if I can provide some helpful ideas with this belt problem.
I have seen this before when a fan clutch is locked. If the fan load is too high due to a faulty fan clutch, this will trouble the belt. If you hear the fan roaring when the engine is reved up when hot the fan clutch may be bad. With the engine hot and turned off you should be able to grab the fan and turn it by hand easily.
Contaminants on the belt is another common cause of belt failure. Leaking front crankshaft seal, coolant leak, leaking power steering or power brake unit, leaking master cylinder. Fix all leaks because belts can not tolerate contamination.
Alternators are not designed to charge dead batteries. If you have to jump start your RV often due to dead battery and then make the alternator charge a dead battery you will eat up belts. Repair battery draw down faults or install battery disconnect switch. Replace battery if it does not hold a charge.
Wobbly pulleys due to physical damage or faulty water pump causing pulley to wobble will make the belt come off. Remove belt and check water pump turns freely and shaft is not loose.
Pulleys out of alignment due to wrong parts, or improperly installed parts. Belts must travel in perfect alignment. Visually inspect belt alignment, use a ruler across pulleys to check alignment.
Use high quality belts. like Gates, Dayco, Napa, Carquest. Some brands of fan belts on the market are poorly made, make sure you're getting the best.
Be sure belts are properly tightened. Recheck belt tightness after a few hours of use after installing a new belt.