I made one up for my old 14.44.3 you can find info somewhere in my old thread
http://www.expandasdownunder.com/threads/drovers-14-44-3.4502/ ..
The one for the 14 would hold about 40lts I think which was enough for 2 days of careful water use, depended if you used the shower really, while the one I fitted to Big Mal holds 90 lts, with both systems all drains ran into the tank, to drain normally just hook up drain hose and turn on tap, for tank holding leave tap closed, no back up into shower problem if full, as it will flow out of the air breather under van.................you can decant into a bucket easily to dump and extend the tank life......................The big plus is by removing the end cap you can flush out all the goobers that accumulate , an added bonus is the complicated plumbing Jayco set up to stop back up is disposed of and your plumbing becomes simple.
The theory of matching your storage is great on paper but finding the room may be a problem, I would suggest go to a van park and use your water sparingly from the tank and actually see what and how you use your water, if you like to have 10 min showers then you won't be happy free camping, it all depends on how much you really need to use which in time the lesson on water use relates to how much you can carry and how much time you want to spend resupplying your tanks............
Store bought tanks can have drain water inlet and outlet problems, the one's I have seen are mostly just water tanks......buying a canister type is a lot of money for little capacity.