This is the service-port pick: Ronco's 6 inch inspection lid with both collar and lid in solid white. Six inches admits a hand and cleaning tools, which makes it the practical minimum for a tank you intend to scrub rather than merely observe - and the opaque lid seals the port into an unbroken white surface that keeps light out of the water between services.
Choose it over the clear-lid variant when the port faces the sun, when the tank sits somewhere a lid will get scuffed past transparency anyway, or simply when you want the installed hardware to match a white tank so closely it reads as part of the molding.
Ronco installs the collar on your tank at its Tustin, California plant at order time - per-order fabrication being the house method since 1974 - so the assembly ships finished and leak-checked as a unit. It pairs with tanks across the RV water tanks and RV holding tanks ranges; see everything on the Ronco Plastics hub.
OneSource Plastics is an authorized Ronco Plastics dealer.