I too am going through this right now..I unfortunately didn't salvage a wire from the front end near the battery box back when our floor was rebuilt and the interior walls were pulled away.
I purchased a super lightweight Lithium Iron Phosphate Battery made by Battery Tender. It only weighs a bit over 3 lbs and it is only 6"x6"x2" and has the power of a large Motorcycle/regular car battery and has tremendous lifespan capability and charging ability. I have tapped in to my 12v
system under the sink area and routed one wire down under the trailer out the hole for one of the copper propane lines and guided it back up and into the Refrigerator area (again, through one of the grommets for propane plumbing) to service the fridge..
I've also tapped into the 12v
wiring from the bathroom closet area up near one of the bedroom 12V
light fixtures. This way I can route this battery or even another one (to add capacity) in the closet floor, under the bed, through the wall under the stove and sink, and then over along the wall to the front where I'll install our new fresh water tank and pump in front of the sofa under the front shelf.
Basically now I have new 12V wiring down low all over the trailer and have flexibility. I have no idea where in the world the wires for the front and near the fridge area dissappeared to..the remaining ones are all just around the upper cabinets in the kitchen and dining area upper light and then the bed lights. But now I have a battery perch above the sink, for one, and under the fridge, if I want, along with back in the bathroom closet. I'll use the outdoor exposed wire down by the propane plumbing line as a "tap in" point to run a charging wire from a solar panel. And of course I can always also charge the system with a charger from one of the 110 outlets, in addition to the vehicle RV plug (which I'm pretty sure still works, I have to research that some more)
I'm a complete electrical amateur but it has been fun kind of fumbling my way through this process routing up some new access points. I've run this little battery, though, from the kitchen overhead cabinet where I tapped into the 12 fixture and it is amazing how long it lasts!