So I have an idea! Let's see what you guys think of this mod. I am going to convert the vented battery door to a solid door which I have done already at the sheet metal shop. A whole $2 for the piece I need to fill the vent. I trimmed it down to fit and will caulk it in place tomorrow.
Next I am going to cut a large hole in the left side of the battery compartment/tub with a roto zip. This will allow me to fit 2 larger AGM batteries inside the compartment and also keep them warm because now they will be inside the camper. I will also stuff 1 AGM under my fridge as there is a compartment behind the wall. I will insulate the exterior access door and may even caulk it shut to keep all water out.
I am thinking I could fit something larger then 31's in both places. Total of 3 batteries and they need to be the same size. I will try to post some pics this evening of my plans.