I'm thinking that you ought not hear the thing inside or outside of the camper unless it is VERY quiet and your specifically listening for it.
So if it is actually a 'fan' ether the bearing is about to go out or more likely something is interfering with the blades causing the racket.
Does your camper have any other fans? (bathroom? fantastic vent?)
My old camper had a failing AC/DC converter which would cause the fantastic fan to have a 60hz "humm" when plugged into shore power. Replacing the AC/DC converter got rid of the noise - however I surely could not hear the "humm" in my house!
If it actually is a compressor fridge again it ought not be any louder than the one in your kitchen!! Something is wrong for it to make that much noise.
M.