Don, I recently transferred my ten to a van. Here is what I did.
I needed a small footprint and searched until I found this tank:
http://www.plastic-mart.com/class.php?item=913
It has an 18" footprint. I wanted 150 gallons but two of those tanks mounted behind the front divider worked best.. didn't interfere with my tool shelves and still left enough room to get in and out easily with my hose reels mounted at the side doors (diagonally). You can drill two 2x4's through the floor as rails and use eye hooks and ratcheted straps to hold them in place.
As far as supply issues.. ten gpm is a big pump. I am lucky to have good municipal around here. About one out of every 50 jobs the tanks can run dry on a deck job. On house washes there have been no issues. It would be prime to have a bigger supply in the 200 gallon region but good luck finding something that doesn't eat up all your real estate.
Over the winter do some drawings of possible layouts and when you think you have one, make carboard cutouts and do a mock setup to make sure you will have the clearances you need.
PS: If you go with the multiple tank option from Plastic Mart, call them, they will cut you a break on shipping. I think I paid somewhere in the range of $290 for the two delivered and had them in 5 days.