Here are pics of my battery box mod. I used a $35 tool/storage box from Lowes designed for the back of an SUV. It's bolted to the same plate as the original box. I had to move the propane tank mounting back a couple of inches to get it in behind the tongue jack.
When these interstate batts wear out, I'll replace them with the 6 volt variety.




I added the switch in case I need to disconnect the batts. The extra space in the box is used to store the wood for jack stand pads, tie-down bungies, the tongue jack foot, gloves, rope, etc.
Edit for SPELL CHECK :S