You are not restricted to the number or size of batteries that will fit in your battery compartment. You can take along one or more batteries not connected up to anything, and when you get there, haul them out and set them on the ground next to your battery compartment.
Connect them up with sets of jumper cables to your one installed battery and now you have a bunch of batteries. They will all recharge at once from your converter/generator too. De-camping, just put everything away and haul it all out again next stop.
I did that for one summer with the fiver and it worked just fine. Had two 12s on the ground in parallel with each other and two sixes up in the battery compartment. Used a standard set of jumper cables to parallel the pair of sixes (one big 12) in the trailer with the pair of 12s (one big 12) on the ground. Next year, I put the two 12s up into the front cargo bay for a more permanent solution, but I didn't have to.