I built a box that holds two group 24 batteries between the exterior wall and the propane tanks. I based the design on the original battery box for my 1961
Safari. I think you can see photos of the box if you check under "my images". It was a tight fit, but I have a good 3/8th of an inch to spare. My original box was riveted to the front of the trailer so I used the same method for the new box. That makes sure there is room for the equalizer hitch to fit without obstruction as it keeps the tongue free.
I have been using it for several years and some long trip without any problems.