I just measured our trailer, it has 24 1/2" of useable space between the frame on the front of the trailer, and of course more towards the back. The existing battery box is just at 12", so it appears there would be room for a second battery & box if you wanted. The top of the box on our TT is marked GRP 24.
2014 Chevrolet Silverado 3500HD Crew Cab, long box, Duramax and 3.73, SRW
Equal-i-zer 1k WDH, 2013 White Hawk 27DSRL
Los Madrones, a little slice of Texas hill country heaven