Yea I'm on the prowl for a used wagon for all your reasons...plus I need seats to haul my almost grown up kids.
In my early stages of dreaming though, I wanted to go with a cargo van and install a custom floor with 4 full lengths of L track I could used for 3 single seats and have numerous tie down options for motos. I wasn't worried much about NOT having airbags back there but, I did want windows, HEAT and AC!...I plan to use this van in a wide range of climates and I'd think such climate control, even short of having passengers back there, would be damm nice!
I soon realized that if I customized every aspect of this van, it would take a ton of time, energy and $ So with the help of the awesome forum, I began to simplify my build.
I plan to remove and sell some of the seats, prolly only keeping the 3rd row in place for my kids. I figure I could move the double 3rd row to the 2nd if needed and/or add a double seat. (but don't plan to do this often as these seats are heavy) I'll throw in an aftermarket floor liner to protect the carpet, but I'm unsure if the stock floor can handle the weight of a 380# moto, we'll see. If not I can always install a custom floor later AFTER my bike has effectively crushed all the under floor heat ducts. I also might add some plywood to cover the rearmost windows BEFORE the handlebars find their way through them.
The first big mod will be a platform bed where i have found endless ideas here.
Then I'll remove the plastic and headliner and install both insulation and simple electrical for a house battery, which I'll locate somewhere in the support boxes for my bed platform. I've read it could be hit or miss getting the headliner back in correctly. So, IF it doesn't keep in place, I may secure with 2 rows of L track. Don't plan to install upper cabinets since there are enormous cooling ducts up there, but instead use my trusty duffle bags and modular camping equipment boxes.
Anyhow, I hope this brainstorm of mine helps you think it through.
Good luck!