Both the belt and tensioner are application specific. The tensioner has a very high spring rate. When the belt length is correct and the belt has not "stretched", the idler should maintain the proper tension.
These idlers are on the "slack side" of the drive. In that position, they do not have to maintain a high tension statically. The spring loaded idler is the secret to making these drives work. With them the tension in the belt is proportional to the load (you don't want to hear the technical explanation
- but I can refer you to an ASAE paper I wrote that gives the engineering explanation
). Since the belt sees a rather low belt tension over most of its life (only high tension when one or more of the accessories are working hard), it does not stretch and wear very much - thus the high rate spring works fine.
The idlers generally work pretty good for a long time. Very infrequently they will develop some high internal friction, but most failures are tension spring. The idler bearing fails once in a while, but that is different from a tensioner failure in my mind.
I say that the tensioner is application specific. A subsidiary of Gates made tensioners for the automotive OEM market and was a major supplier. They then designed a tensioner for a Detriot Series 60 engine and had major problems. When the engine would idle (think of a truck parked over night), the torque pulsations would make the idler dance and caused premature spring failures (think a month or two life). We set up a brand new Series 60 in our lab and let it idle 24 hours per day. Finally got a good design.
My guess is that the someone had replaced the belt on the motorhome with an aftermarket belt that was too long. That let the idler get out of its fairly narrow operating range.
Sorry, tired to avoid replying to this thread, but my geek side would not let me