1. Do you have a slanted vent on the OUTSIDE wall of the MH for the range fan to exhaust?
If you do check under the vent. On my first used MH I found the previous owner stuck a piece of foam in it to prevent it from flapping when driving down the road. Which is why the fan wasn't sucking air properly.
2. If you do 'not' have a vent on the outside. You then have one of the newer range hood fans that only sucks the air thru a filter in the hood. It doesn't exhaust outside so the drawing effect in the MH is no where's near the same as a vented one.