During the RV manufacturing process all kinds of things get into the holding tanks: dirt, sawdust, plastic plugs (shown here),
electrical wire pieces, screws, bolts, staples etc. These items should be blown out of the tanks prior to water testing, as some of them can stick to the bottom of the holding tank.
This particular piece, came from a hole saw falling into the tank during tank assembly. Unfortunately this plug made the tank, drain very slowly. It was found in a 3” elbow after the Drain Master valve. This same plug can get caught in the tank outlet before the valve and the result, is the RV owner thinking the valve is not working because the holding tank is full and will not drain.