You will need to either make a sealed and vented box in that location or buy one. You can't use the typical black plastic battery boxes and vent the area that they sit in. If you proceed with that, you'll have the explosive hydrogen and corrosive hydrogen sulfide free to work it's destructive magic. One spark in the area when those gases are present can ignite the gas mixture.
You should also do some side-to-side weight measurements to make sure this is a viable weight transfer.
FWIW, I have two GC2 6v's inside a box located just in front of my axles, inside the cabin. The box I made for the batteries is gasketed/sealed and vented to the exterior.
You may need to upgrade the wire to the tongue to operate the jack but I don't know the specifics of the load and your wiring. Fuse the positive lead from your new battery location at the batteries.