Seventy-one gallons is not a round number, and that is the tell. Moldings like the RVB531 exist because somebody's bay had exactly this much room, and Ronco tooled the shape that filled it completely instead of leaving gallons on the table with the nearest round-number tank. That is the B-series philosophy in miniature: the catalog is a library of real compartments solved, accumulated since 1974 at the Tustin, California plant. If your bay matches the technical drawing below, you get every cubic inch of it working as water storage.
The tank is light-duty translucent natural polyethylene, FDA compliant for drinking water, 111 pounds as molded. The translucent wall shows the water line without instrumentation, and the natural resin is the honest, uncolored grade you want in potable service.
As with every Ronco molding, fittings are installed per order - spin-welded or threaded to the marks you provide. Print the drawing, lay out fill, vent, and supply against your plumbing runs, and settle the final layout in the fitting planner on this page. Then give the mounting the respect 71 gallons deserves: water is 8.34 pounds per gallon, and a full tank should sit low on a supported floor.
If the drawing is close but not quite your bay, the RV water tank family and the full Ronco Plastics catalog hold the neighboring shapes, and the freight estimator prices delivery. OneSource Plastics is an authorized Ronco Plastics dealer.