It is an air compressor for a RoadMaster/BrakeMaster auxiliary braking system for a towed vehicle. Go to the back of the MH you should see an air chuck by the hitch. An air line connects to this fitting and to a mating fitting on the towed vehicle. From there an air line is routed through the engine compartment to under the dash. An air cylinder hooks to a mounting bracket under the driver's seat and to the brake petal. A Proportioning valve in the brake line of the MH will energize the air line proportionately to the applied braking the the MH. this is one of the best towed vehicle systems out there.
Jeff & Rini, Chazy, NY
2012 Winnebago 33C Sightseer, Ford F53
2012 Chevy Silverado (toad)