If I recall correctly the Lance cord is specifically designed so that enough truck-to-camper current is available to run the fridge on 12V.
In some states its ILLEGAL to drive with a propane appliance actually using propane. If you recall your "walk around" when you bought your TC (this IS a TC forum after all) the dealer may of mentioned that your propane tanks ought to be TURNED OFF while the camper is in motion. (at least both of mine told me this)
With that said, I'm guessing that your battery needs to be replaced.