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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.
You can have one belt on a common pulley experience problems when other ones don't.
Belt load, and distance traveled comes into play. There is a long distance between the alternator and the crankshaft pulley on one side.
When a belt on a common pulley travels a long distance with nothing touching it, it can start flapping and jump ship.
Careful inspection of all elemenets listed above should reveal your problem.
Groovy! Good luck!
The green belts you are talking about are made by Gates and they are called Fleetrunner Heavy Duty Belts. They are made for fleet trucks with extra high heat capabilities. These belts will ride a little higher in the pulley because they are slightly larger width being a true 1/2-inch belt, so you have to size up one size from your current belts.
Your 15495 belt would take a 9500HD Gates or a 259500HD NAPA.
Your 7585 belt would take a 9600HD Gates or a 259600HD NAPA.
As for where you can buy them, a truck parts place or a NAPA store. Most places do not carry these belts on the shelf so you have to order them and wait a day.
If you want to buy them on the net, you can order them from acdelco.com just use the gates numbers listed above.