I just installed a 2nd batty under the hood on the passenger side, and it gets me enough amp hours to run a ham radio, chargers, and ARB fridge but there are times I want more run time for extended runtime.
Since there isn't really a good place for a 2nd house battery, I ended up with two different solutions that work pretty well.
1. I use a MPPT solar charge controller and a 60w folding panel which is all stored in the back in one of those compartments.
2. (And or) I have a 3rd battery box which Anderson Powerpoles which connect in parallel to the house battery circuit. I can actually plug this in a few different places, but I generally keep it in the cargo area.
The 3rd option, which I dont have yet, is to add another bank in a trailer.