In our toyhauler I have a bank of (4) 6volt batteries so we're used to lots of juice. We just picked up a used 1999 Lance camper capable of carrying only one 12v battery. I wanted to at least have (2) 6 volt batteries....
Here was my solution: I built a vented battery box and placed it under the kitchen sink adjacent to the existing battery box.
The box is all sheet metal and the interior was silicone sealed at the joints. I painted the sheet metal and I used marine grade rubber weather seal for the top plate which will be held down by commercial grade Velcro straps.
Inside the sheetmetal box I've put the bottom part of a plastic battery box just in case there is battery leakage. I will be sealing the wire exit hole with silicone.
The 3/4" plastic tubing is from the hardware store and I will be using #2 AWG wire to put my (2) 6 volt batteries in series.
All I need now is to get batteries and the make the cables with the correct size wire...