Could use a bulkhead battery connector as used on sealed battery boxes. Can use a boot over the terminal on end of wire like in 2nd photo if needed. I used these on a sealed battery box with lead acid battery in a previous TT. An AGM battery would be a good upgrade but should still be vented as they can vent gas if they get overcharged.
MTS Products makes an inexpensive sealed battery box for RVs. I was in the parts section of an RV dealer once and they had a stack of used ones for sale at the back for $5. Got a couple of them that looked like new. These boxes are used in FWs. Has a built-in gasket and a flange on top and bottom.