Modern RV toilets sit on a raised floor flange. A single raised floor flange is required to create a watertight seal. This 7/16" thick ring is just like any floor flange (or closet flange) except that it doesn't have the funnel portion that mates into the black tank. It can be stacked on top of an existing flange to build up the height of a flange when adding new flooring on top of existing flooring. Or it is a great problem-solver for vintage trailers that originally used toilets that did not use a raised floor flange (like the 1963 Airstream shown).
ID = 4-1/4"
OD = 6-11/16"