It's not an underfloor H2o tank solution but you may want to consider
SODA KEGS, 5gal SS cleanable, pressurized liquid supply tanks,

mine will hold either homebrew or H2o depending on the nature of the
trip/camp

Item number: 7621541680
<http://cgi.ebay.com/5-Gallon-Cornelius-Keg-Beer-Soda-Homebrew-2_W0QQitemZ7621541680QQcategoryZ67137QQssPageNameZWDVWQQrdZ1QQcmdZViewItem>

these can be run with a small CO2 cylinder for pressurization.. no 12vt
pumps,

dan in syracuse

From:    David Wilhite <dlwilhite@COMCAST.NET>
Subject:

Guys,

I am still needing a water tank after changing my Westy to a 7
passenger keeping the stove and sink intact.  My preference is to put a
tank under the floor, but I have been having difficulty getting a tank I
can afford that will fit.

I have seen the Reimo website, but from what I understand the shipping
costs would make it cost prohibitive.  Eurocampers carries one
(~$95.00), but they don't have any in stock and they may or may not get
some soon.

There are plenty of polyethylene water tanks on ebay at good prices
(~$40-$80), but none of the standard sizes will fit.

Of course, I can get a custom made friction-welded tank for $400, but
that's way out of my budget.

A final option would be to pull the fridge and install a water tank
there.  I do not want to take this route, because I do use the fridge
occasionally on camping trips, but I may have to.

Anybody have any other sources are some other ideas of how to add a
water source?

Thanks,

David Wilhite
84' Westy
Middle TN