Here is a picture of my conversion to two six volt golf cart batteries (Trojan T-105's). I had to cut the white plastic trays to allow the taller batteries to tilt enough to install, but I didn't have to cut them all the way down to the bottom as you can see from the photo. Please note the wiring was very simple and actually used one less wire than if I was hooking up two 12 volt in parallel. I used both white leads as you can see. The original leads from the Designer aren't visible unless you look real close in the shadows, but they are totally original including length.
For what it is worth, there is 1/8" clearance between the top of the top battery and the on/off switch. Close, but it does fit.
The tie down straps were the original ones provided by Jayco.
2006 GMC 2500HD CCSB 4x4 Duramax/Allison, Titan 52 gallon fuel tank, Prodigy Controller, B&W Companion Hitch
2010 Jayco Designer 35RLTS, Cummins/Onan RV QG 5500 EVAP, Progressive Industries EMS-PT50C, TST Systems 507 TPMS, RV Flex Armor Roof