Some photos of your particular set up would be really helpful in collecting ideas. My own trailer had only room for a single battery also as the LP tanks were right against the front wall, putting the battery at the point of the tongue. I was able to remove the existing battery and fill in the tongue with a 2X plank, making it a level platform. This allowed me to rotate the battery 90 degrees and add a second one right next to it. Both boxes are screwed down to the plank which is in turn bolted to the trailer frame. Straps are used only to hold the lids on. I've never seen any RV that could not be modified to handle a couple of batteries.
Good luck / Skip
