Great idea with the tool box battery case...
I have been doing something similar with a couple of GP24 battery boxes mounted on a small one inch panel in the back of the truck bed. I have a long 4AWG two conductor cable that will plug into my off-road POPUP trailer. This all temp for now but will eventually wire all of this into the truck permanent.
This supports my Ham Radio ops for the truck when by itself and will also add to my POPUP trailer when in camping mod.
Adding battery bank to the truck bed was big brain storm since my 2KW Generator is already sitting there. Takes weight off my trailer tongue area and keeps my Camping and Ham Radio hobbies going...
My ultimate plan was to install two TROJAN T1275 150AH batteries in both locations... That would give a big 600AH battery bank when in camping mode.
I am currently running two groups of 12VDC 85AH Interstate batteries giving me over 300AHs of usable batteries.
Roy Ken