What could be causing my belts to break on my 1983 Shasta motorhome with a 454 Chevy engine?

Forcing the belt on can damage the load carrying cords and cause premature failure. Are you sure you have the right belt? Installing the wrong type belt, tightening the belt too tight, or too loose, or pulley misalignment will all cause it to fail. If it has an idler pulley, it may be bad. If it has a tensioner, it may be bad. Maybe you routed the belt incorrectly. Yes, it is possible to get the belt wrapped around all the pulleys in what seems to be the right way but have it wrong. Check the belt-routing diagram, which on many cars is on an under-hood label. Make sure all the drive belts have the correct deflection. There is a remote chance the water pump is faulty. Try removing the belt and spinning the water pump by hand. It should spin free with no noise. Correct tension is critical to the belt performance. As I do not know what type belt we are talking about, contact your local Chevy dealer and ask them what the correct tension should be.