On the diesel OM60x motors, if the belt tensioner breaks, the entire timing case can get torn up and the engine is more or less done for. You will be screwed. Get that taken care of ASAP - especially before you take that road trip.
Quoted from mbz.org:
"...check that the serpentine belt idler pulley (right below the horizontal expanse of fan belt at the front to of the engine) MUST be aligned parallel with the belt - if it's a degree or two cocked, then it MUST be repaired..."
Note the emphasis on MUST.