Way back in our popup days I needed a bit more storage for hoses, cables, etc so I built a
custom storage box that I mounted at the rear of the trailer. I built it with plywood, designed a locking capped lid for it, and finished it in black vehicle rock guard textured spray paint. I towed that trailer over tens of thousands of miles for several years in just about every type of weather imaginable and it proved to be completely watertight. There's no reason you couldn't do the same to build a tongue mounted box to the exact dimensions you want, my only caution being that I personally wouldn't put a battery (or batteries) in the same compartment as the propane tank(s).
