I've added a house battery to a couple of vans. (one started out as a GLS).
I'm personally not sold on the house battery system as most people use it.
What would you like to power with it?
I fully agree that people need a way to power lights, radio, and recharge things like phones/tablets, etc without draining their main start battery to the point where they are stranded.
Personally I think most people greatly over estimate what they *really* need to power. So they build big expensive house battery systems withe big inverters, and lots of wiring. A system that takes up lots of space and time and labor to install.
Then, when its all said and done, goes unused 99% of the time they own the van. Letting the expensive battery just waste away.
As for the mounting location. There is sorta room under the driver's seat. Ideally you need a Weekender drivers seat base as it has a battery tray built into it. Then the special battery, which really only fits until you put a swivel on the seat, which some batteries don't fit under. They really don't fit when you realize the seats are too tall with a swivel and you want to cut the seat base down 1.5"
I mounted the batteries under the back seat. You can fit a far larger battery under there. Put it in the space that is almost impossible to access.(far forward under the seat).
Here is a great link on a cheap way to add a house battery....and solar.
http://www.expeditionportal.com/forum/threads/77503-How-to-make-a-cheap-isolated-dual-battery-setup-for-50I've been playing with all-n-one "solar generator" battery packs. I know many people on the net like to argue that they don't have the total capacity per dollar of a DIY system like the link above. But they also have versatility and easy of use that those systems don't have.
You could easily add a plug to plug a Goal Zero Yeti (for example) into the "house functions" of the van. It would charge when driving and provide all your power needs when stopped. Aldo allowing you to just take it with you for other adventures away from the van.