This is not an unusual problem and I had the same thing happen to my 2010 Interstate. The cause was that the foam gasket that is intended to seal the vent when it is closed (pulled down) came loose and was sucked into the fan blades preventing them from turning. The fix was to remove the vent cover from the top of the van and remove the old gasket material including the pieces stuck in the fan blades and install new a new foam gasket made from household weather strip material.
2010 Interstate 3500 Twin Bed
(2008 Freightliner CRD Sprinter)
TAC AK-1 !!