Have you considered those heating pads that stick to the side of your water tank? You plumb the electrical into a manual switch of some sort, but my father in law uses these in all his RV's and he has a Mercedes Sprinter that he installed these on as well. Actually the Mercedes I helped install, which was very easy. You simply stick the pads onto the tank, run one line to 12V ground and the hot through a switch. Didn't take long to set up at all.