Either ABS or PVC could potentially be 'glued' to make tanks. For fresh water tanks it's important to get 'food grade' material. Polypropylene must be welded but it is the most economical food grade material with desirable physical properties such as impact strength. Learning to do the hot air welding takes a day or so but it can easily be mastered. Craig shepard has an excellent discussion of building tanks at. http://www.gumpydog.com/bus/MC9_WIP/Plumbing/Holding_Tank_Fabrication/holding_tank_fabrication.htm
I built 4 tanks following (sort of) these instructions. I have two fresh water tanks totaling 180 gallons, a waste tank at 220 gallons and a coolant tank for my Webasto at about 5 gallons. Email me off board if you choose welded tank construction.
Jerry 4107 1120