There are obviously several areas that can cause a belt failure... obviously as stated above, look for a miss-alignment concern.
How are the belts failing? Snapping ?? excessive wear on an edge causing a fraying issue ?
Depending on hours, CHECK THE TENSIONER !...if you see a rusty type powder residue coming from where the arm and base meet, the inside is shot...if there is excessive play on the arm side to side, it's shot... work it through the motions as well and make sure there isn't a binding or the spring has lost proper tension. Most of the above mentioned tensioners are easily identified by a section of the "flat" spring sticking out of the side of the base.
Also appears there are two belt options depending on year.. I belive the split is @ 2003.
Case# A77938 = Gates K080610HD pre 2003 ?
Case# A78022 = Gates K080790HD aft 2003 ?
The HD designation at the end of the belt is the FleetRunner line with a premium construction.