On my Wildwood 26TBSS I bought a metal box from Harbor Freight, it fit between the frame of the tongue (where your current batteries are). I carried 4 six volt batteries in it. Maybe something you can take some measurements and see if this works for you.
Dale & Terri, Lulu & Tiki (our Chihuahua's), New rescue puppy Prince - Pom/Pug mix.
2013 Tundra, Double Cab, 5.7, TRD Off-Road, 4X4, Full Tow Package
2013 Wildwood T26TBSS - Sold
2000 Gulf Stream Sun Voyager M-8357 MH, Ford Trident V10 Gasser, 35 foot.