Hallo Simmo,

On my 1990 W124193 ( 603 turbo ) it's lasting 375000 km now and still in good shape.
In winter at temperatures lower than 0° C, the belt makes some noice.
Than I put some chalk on the inside and the noice is gone.
In your case it's certainly not normal that the tensioner is gone that fast.
Let us know about your sollution when found...
Good luck,

Danny