Oh well, sorry for wrong advice, but you found it.
While you have the belt loose, you might push/pull on the tensioner pulley and be sure it moves smoothly and has a lot of resistance in both directions to movement by hand (may need to tighten the allen bolt again). All it is, is a big rubber doughnut that flexes to maintain tension. But when they fail, you walk home. Actually, if you have that many miles, you maybe should just replace it on principal - the only time in 18 years that my MB failed me was when the tensioner failed last summer 90 miles from home - no warning.

Also, the guys are right about the voltage regulator. They seem to be the most common fail mode for the alternator. They cost less than $30, and can be replaced in about one minute with two screws. They include new brushes.
So, if your brushes look badly worn, but the alternator bearings and commutator still seem okay, you might just pop a new regulator in and see.

DG