I used an ATV tool box as a tongue box and for the first six years of my trailer’s life it was also a battery box.
I made wooden dividers to keep the batteries from shifting around, along with an upper and lower louvered vents on the back.
The 2 gauge cables went through outdoor air conditioner conduit, which seals to the box and trailer. I never had any water in the box. You can barely see the conduit in this pic.
3 marine deep cycle group 24’s. These things were heavy!
Last year I upgraded to LiFePO4 and moved it indoors. It’s nice to have outdoor storage in the tongue box for tools and 1 lb propane bottles now.