I used 8" PVC drain pipe for a holding tank. The construction sequence is here http://www.skoolie.net/gallery2/HoldingTankConstruction
I don't know about using it for freshwater. If I had it to do over again, I'd go with a larger diameter pipe. My tank holds about 25 gallons, but 12" pipe the same size would hold 50 gallons. On the other hand, my tank is graywater only, not blackwater, so I could pretty much dump anywhere in an emergency (no, it wouldn't be legal everywhere, but I did say "emergency").
If you can work a regular tank into your scheme, I think you'd be better off. Net cost would be about the same, plus much less work. One thing you might consider, if you have an odd shaped but high volume area, is a bag tank like they use on boats. Look at boating supply sites.