A tank you can see into is a tank you actually check. This is Ronco's 4 inch inspection lid in its white-collar, clear-lid configuration: the threaded collar is installed on the tank at the Tustin, California plant when you order it with a tank, and the clear lid lets you confirm water level and condition at a glance without ever breaking the seal.
Four inches is the compact port - large enough for a look, a light, a sampling dip, or a wand rinse, small enough to fit tanks and mounting faces where a larger collar would crowd the fittings. The white collar sits naturally against Ronco's natural and white moldings, which is why this combination is the default on fresh water installations.
Order it on the same line as the tank it belongs to and it arrives already installed, positioned along with the rest of your layout - Ronco has been doing exactly this kind of per-order work since 1974. It suits tanks across the RV water tank and boat tank ranges alike; see the whole accessory and tank catalog on the Ronco Plastics hub.
OneSource Plastics is an authorized Ronco Plastics dealer.