Want to add a battery to my truck camper - help please

Hmm run ours on 12v on the road since 2001. Ive done a few different mods making it easier, switch off/on from inside truck, upgraded wiring etc. It is the least effciant source. Personally I wouldnt consider a non 3 way. But it does require diligence or setting up to use. But I think the main topic is adding a battery. What OP does with it?

As to the TL mount it clamps to frame. Fairly straight forward design, on the ford its on passenger side.