I believe I read this post or one very similar before I bought my 2011 Wildwood 26BHXL and solved it, able to have two industrial 12vt batteries. The need of two big batteries was a must, I love to dry camp away from the many, not so much the wife she’ld like a shower every day.
Back to installing two batteries: I cut out the original battery frame and carefully removed the double propane tank base. I then built(welded) a battery frame 4 inches lower than the campers “A” frame and several inches further back(this still keeps everything above lowest point of tongue jack). To fit both batteries they had to go, slightly, under the propane base so it was raised 4 inches and it partially covers battery-1 by 2 inches. Now I designed it so the covers of both batteries can still be removed with little interference. Also when I need to get both batteries out I take battery-2 up and out, slide battery-1 back to where battery-2 was and it’ll come out. The propane tanks did not move font to back just up, the propane line was sufficiently long but it is something to be aware of. Propane tanks could not be moved further forward do to the interference with the tanks cover and tongue jack.
I cant find my pics but will take new ones and post again
Took some new pics
Good luck and
Keep the shiny side up