If you are hooked up to the motorhome lights properly, you should have 12 VDC in the dinghy. Your coach probably has a bank of 2 or 4 6 volt batteries for the coach and possibly for the chassis. It might be possible to have hooked the lights up incorrectly, but highly unlikely.
Also check the connections and ground for both coach and the dinghy as well as the cable.
Amateur Radio Operator (KE5DFR)|Full-Time! - 2012 6.7L Ford Crew Cab Dually -2013 HitchHiker Champagne 38RLRSB - Travel with one Standard Schnauzer and one small Timneh African Gray Parrot