SOUNDGUY - They sure make it hard sometimes huh...
I do almost the same thing here that the OP is doing. I have two GP24 batteries sitting next to my truck tool box in my truck bed. These are to support my Emergency Radio Ops but neat having a couple of working extra batteries for the trailer I guess...
I could run a two-wire drop from the back of the truck and install a second cable between the truck and trailer and use my extra batteries if needed. Shouldn't be too hard a deal to make up...
There's always a way to get around these problems
Roy Ken