I think Javi is correct for a Ford, for GM products there is a wire under the hood that you have connect in the fuse block under the hood. Between the left (driver or right side as you are looking under the hood) fender and the large fuse box is a wire that says "accessory" I believe, it was red on my last 2 GM products. It gets hooked to a stud that is under the large fuse box. Once you connect that, it should charge the battery from the TV. See this info from etrailer.....
http://www.etrailer.com/question-21543.html