When we added solar, we converted the house battery bank to 8 batteries -- far too many to squeeze into the existing battery tray. Instead of sacrificing a basement compartment to host the newly acquired amp hours, we had a shop in Denver fabricate a battery box that fits/sits inside the frame rails -- dead-center in the chassis. It is a bit of an inconvenience to slither under the coach twice a year to check on the batteries, but I am skinny and once I get a drop light in there it is quite roomy (lots of wasted space down there) and pretty easy to check battery water levels and connections, etc.
This battery relocation idea is often met with resistance (not ohmage) because the space cannot be accessed by sitting in a lawn chair. I'm a pretty agile fellow and I liked the idea of getting the batteries located low and central (and closer to the charger and inverter). I also liked the idea of freeing up the compartment's OEM battery tray for things that I need to access more often than semi-annually. And I liked the idea of having a comfortable 12" of work space above the batteries.
The cost of the battery box was about $250.00, as memory serves. It is bolted in place and does not move -- there are no battery connections that get torqued by sliding a tray in and out. My version is heavy steel, though I met someone who built his out of aluminum. I had the option to powder coat my battery box, but opted just to have it painted black, instead. Had I to do it all over again I would opt for the powder coating (I am starting to see surface rust in a couple places on my box).
Food for thought.... Relocating the whole battery box might work for you, but it might not, too.
PS. I don't have any pictures, but one of these days I will remember to take a camera with me down there so I can post some.
Ken & Deb (WIT Club, FMCA, SKP, Harvest Hosts, and grateful volunteers with SOWERs)
2005 Winnebago Vectra 40FD w/400ISL and Allison 3000MH on Freightliner's Evolution chassis, Onan QD7500, 1000w Renogy solar, BlueOx tow gear, and 2015 Cherokee TrailHawk toad.