At 9:10 RVFD was alerted along with other units to a vehicle fire threatening structures at 14650 Union Rd. in the 7-9 Box.  Eng.401 (6 w/Chief Utz) and Tanker-406 checked up on the air within two minutes.  Eng.703 arrived on location with a trailer/camper in the rear of a dwelling with smoke showing.  Crew of Eng.401 arrived and went to work with the interior crews extinguishing the fire and performing salvage/overhaul operations.  Tanker-406 supplied Eng.703.  The incident went control and the Maryland State Fire Marshal was notified.  The fire appeared to be electrical in nature.