Yikes! $45000!
Any way you could buy a van of some sort and deck it out with the stuff you need? I'm thinking used Toyota Sienna for $8000 or so (6 months ago we spent $8200 on a 2009 with 112000 miles on it). You could fit a top quality twin mattress in there and still have room on one side. With proper shelving you could have storage space under the bed frame. I bet you could find someone to rig it up for you including a/c, solar, basic kitchen, etc for much less than $30000. And a minivan would be way more stealthy than a sprinter van.
+1 on the suggestion to rent something like this before dropping $45k. Borrow a friends van/minivan or rent a construction van and throw an air mattress in there and try it out for a few nights.