The mechanic tasted the water that pumped out of the Onan oil reservoir and said it was freshwater. He also tasted the water in the muffler and said it was salt water. Fresh water is less dense (weighs less) than salt water, so it will float on top unless there is some action that causes them to mix. It's entirely possible that the fresh water floated on top of the salt water in the muffler and when we drained it, with the drain at the bottom, the salt water came out first.
all this aside, it's clear that the flush valve leaks into the sea chest: I removed the sea chest lid, closed the sea chest thru-hull and waited. The water level in the sea chest steadily rose during the day.
Many thanks to everyone for their help, especially Bob from Rossi Design Group...this forum is truly a blessing!