Here is a 45 gallon class fresh water tank molded for the long haul: the Ronco RVB318 ships at about 90 pounds, wall thickness you can feel when you knock on it. Rated light-duty for potable fresh water, it is intended for the drinking water system of a motorhome, travel trailer, or specialty coach conversion.
Two things make it an RV-grade potable tank. First, the material: natural FDA-compliant polyethylene, no dyes, translucent enough to show the water level through the wall. Second, the construction: rotational molding produces the entire tank as one seamless unit, so there is no joint, weld, or seam anywhere for road vibration to attack.
Ronco Plastics has been rotomolding in Tustin, California since 1974, and it finishes each RVB318 to the buyer's plumbing plan by installing spin-weld or threaded fittings at customer-specified locations, its standard for every tank. If a drawing is shown, it depicts the blank molding before fittings are added. OneSource Plastics supplies this tank as an authorized Ronco Plastics dealer.