Drain trap for the grey water plumbing to the exterior drain connection.
This trap is commonly found on mid sized and larger pop ups, and attached to the inside of a side panel.
Measures about 10 inches in width as pictured. Accepts to a 1-1/4 drain hose via the male barbed connection. The threaded connection on the right side is what you see on the outside of the camper, and you attach the drain connector to.
Part number 4769A6941