If you get the stinky smell only when you flush then it is from dead decaying organic matter in the flush line.
You can put in a strainer filter next to the seacock.
You can flush with fresh water from the sink.
Use fresh water flush when you leave boat to sit.
People say they use the drain line from sink.
Connect line from head intake to sink drain seacock, only can do if it exits underwater.
close sink seacock, fill with water, flush head.
I have thought about using the heads intake seacock as the sink drain and doing that too.
would that cause the head electric motor
to suck air?