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So you'll just mount a battery box to the tongue, drop your battery in, and run a white wire from the battery negative to the trailer frame, and a black wire from the battery positive, to what ever color wire feeds your ceiling light fixture. You may make this connection at the camper's converter, if you do some electrical testing to see which wire at the converter is outputting 12v to the ceiling light and porch light (if you have a porch light).
Use large diameter wire, like 10ga or 8ga.
Typically, people set up their tow vehicle to charge the camper battery, and run the camper fridge, on 12 volts while towing. To do this, you run an 8ga wire from the tow vehicle(TV) battery positive, through a 30amp fuse, back to pin #4 on the 7pin round bargman connector on the TV. Then, from this #4 location on the 7pin of the camper, you run 8ga directly to the camper battery positive.
8ga would go to the refrigerator's heater circuit too.
8ga would be the size of the ground wire you'd run from the TV ground connection on the 7pin, to the TV frame. You'd do the same thing to the 7pin ground on the camper....8ga from there to the camper frame.
Regarding your ceiling light, this would be a great time to switch to a light with dual lenses, (in other words, it's a double light), and then replace the bulbs with LED printed circuit board versions, like the 9LED "warm white" boards from www.superbriteleds.com. This will make your battery last a loooooooooong time, because the LED use almost no power. Nice feature if you're boondocking/dry camping for days on end while hunting.
Toss a little solar panel up on the roof and it would keep the battery pretty happy.
With the TV set up to charge the camper while towing, you could likewise charge the battery while in camp. Just plug the camper into the TV and start the engine.
If you haven't thought about heat, the Wave 3, Wave 6, and Wave 8 Catalytic Heaters use no battery power. Just feed them propane from the camper's main tank, and they produce silent, radiant heat, all night long. Extremely efficient and they can be wall mounted, recessed mounted, or portable and stand on legs. We've been using a Wave 6 for 5 seasons and love it. Does a great job in our 11' box. We also have the double ceiling lights with the 9LED boards for each of the 4 bulb positions.
For more warmth, or more efficient heating, cut Reflectix Insulation and zip it up into all your windows.