We went to fire up our RV (it got crashed by a tree branch) after 4 weeks in the shop and the condenser runs, but not the blower. It hums but I am scared to leave it run and burn something up. We have it plugged into an adapter and are using an extension cord for power; could that be the problem? Not enough juice? We have hooked it up this way before and had no problems, but with a heavier weight cord (also broken in the crash). :D

What blower is not running? The inside or outside?

Take the cover off on top of the unit. Try spinning the fan by hand with the power off. If tight see if you can lube the motor. If that does not work check or replace the motor. Many times on motor homes/trailers the motor is never serviced and that leads to problems. NOTE you might also check to see if something got into the fan area (birds nest etc).

What blower is not running? the inside or outside?
Inside--the compressor is working and trying to kick but the fan just hums--I'm assuming it's on the inside--that's where the filter is

As HVAC1000 suggested, take the cover off and see if the fan spins, if not then you may have a bad motor or motor capacitor.Good luck, Mike