2000 Dodge Caravan Serpentine Belt and Tensioner

In 2006 my mechanic replaced the serpentine belt and tensioner. This lasted about a year and a half, at which time, after several times replacing the belt, a new tensioner and belt were installed.
This was in the middle of December, 2007.
Since then I have been back to the mechanic at least 3 times for the belt replacement, the most recent being 2 days ago. After those 2 days, the belt once again ended up in the street.
My mechanic is telling me the only sure fix is to install a new kit of some kind with a different belt and some additional parts to make sure it doesn't jump off, at a cost of over $300.
He's also telling me my problems are a result of a "Chrysler design flaw".
The tensioner that was installed in December has a one-year warranty.
If this is indeed a Chrysler design flaw, shouldn't they be held responsible for some of the cost of this "new" kit?
Is this, in fact, the only solution?