It's a tractor battery, David. Still available at farm supply stores or
battery places such as Interstate. If you want to use the box mounted
on the front of the trailer, you have no choice other than to use that
type.
Options include mounting a new battery storage box on the tongue spicing
in a new lead down to it, and using the old box as storage. Try to get
the aluminum style boxes (hot-rod accessory), as the plastic ones fail
from UV exposure (straps & latches). Mounting the battery under the bed
nest to an access door requires a little more work as it involves tie
downs and venting.
Best of luck,
RJ