On my Bigfoot, I installed 2 Battleborn Batteries, I removed the battery box as it was vented to the outside. I replaced the external access cover with a waterproof, insulated cover and now the batteries are inside the conditioned space. There is also a heat duct adjacent to the batteries to keep them toasty. The batteries are partially under the sink so I installed a large plexiglass shield above them and used automotive firewall boots to run the new battery cables through. All switches, main fuse, breakers,
etc, are also protected via plexiglass should a water leak develop and to prevent accidental contact. I'd also like to add 1 or 2 more batteries but do not have the space inside with sacrificing more storage space.