PE is the material of choice for seamless rotomolded water
and waste tanks
...PP has become the material of choice for welded custom tanks
because it's much easier to cut and weld. I used to know of several companies who made PE welded tanks...don't know of a one today (but that doesn't mean there aren't left, only that I don't know any). Wall thickness--at least 3/8"--is very important...then, like anything else, it's the skill of the person making the tank that matters.