Six inches is where an inspection port stops being a window and starts being a way in: room for a hand and a brush, a proper interior wipe-down, and a real look into the corners of the tank. This is Ronco's 6 inch lid in white collar with a clear lid, so between cleanings you still get the glance-and-go level check without opening anything.
The clear lid earns its keep on fresh water tanks especially - Ronco's light-duty moldings are translucent, and a clear port on a translucent tank means the whole system can be read visually, top and side, no gauge required.
The collar is installed on your tank at the Tustin, California plant when you order the two together, the same per-order finishing Ronco has practiced since 1974, so there is no field cutting and no gasket guesswork. It fits tanks throughout the RV water tank line and the boat tank line; the complete catalog is on the Ronco Plastics hub.
OneSource Plastics is an authorized Ronco Plastics dealer.