The holding tanks (there are two) are Dometic HTS-T 28 gallon tanks...
OMG, NOW I know what he has. We're not talking about his toilet, we're talking about his holding tank and overboard discharge pump.
Although the one-way valves in the S-Pump (VacuFlush vacuum pump) and the T-Pump (overboard discharged pump) perform one of the functions of joker valves, Mansfield Plumbing named them "duckbill" valves when they introduced the VacuFlush systems in 1978 and that's what they've been known as ever since. "Joker valves" are also one way valves and are found in the discharge fitting of every manual and electric macerating toilet. See photo.
There's a reason why the valve nipples on the the T-Pump are reverse threaded: it is identical to the S-Pump (vacuum pump) in every way except the S-Pump needs 4 duckbill valves--2 on each end...the T-Pump needs only 2--one in each end--and the price: the list price of the S-Pump is just about double the price of the T-Pump. To prevent people from buying T-pump and adding a valve nipple at each end, they reverse threaded the T-Pump.
I'll get back to the original question Monday morning.
--Peggie