If tightening the nut doesn't stop the leak the best place to have it repaired is at a propane dealer that does residential tank installs and repairs. Most RV places wouldn't have a clue of what to do, besides replace the tank. Or just do as suggested, either have the valve all the way open against the back seat or all the way shut.
Chuck & Teri, 1999 35 ft. Dolphin 5350, F53 Chassis with tag axle, Chassis build date 1/99 in Mexico, Banks System, Trans Command, Air Lift Air Bags on rear, Koni Shocks, Blue OX TruCenter, TigerTrak track bars front and rear, Roadmaster 1-3/4" rear auxiliary sway bar, 2013 VW Passat TDI SEL Premium, 1994 36 ft. Avion 5th Wheel. FMCA #F430129