An alternative to building a box in the crew cab and using AGM batteries is this product from Torklift:
Hidden powerI have one, here, and may mount it on the trailer tongue. I have 5 batteries inside my TT in two banks. Building a good battery box or buying one/installing is not an inconsiderable bit of work. The Hidden power comes with all the hardware but not the hookup cables. I haven't read the specs on the Torklift box but the included Noco battery box measures 7 3/4" X 14 1/2".
An alternative that I'm noodling is mounting a solar panel on the back of the truck's topper and then, I would install the Hidden power on the truck frame. With a short tilt mechanism, the solar panel would also act as a wind deflector when towing. If not towing, the extra battery and solar would be a power plant for off-roading and using the topper as my camp.