"Best manual head under $500"?

Might well be........but just as I won't bother rebuilding a toilet where the
price of a new toilet is very close to the extortionary price of the handful of
rubber parts and pieces sold for several dozen dollars, I'd personally look
very closely at a VacuFlush before spending 5 boat bucks on a hand pumper.

Can't you get into a VacuFlush, (with a brother in law deal) somewhere close to
a geezal? No stinky sal****er in the lines,
much longer service on a holding tank of given capacity, something like one
moving part in the toilet itself, no water slopping out of the bowl if somebody
forgets to "dry bowl" after using.....there's a lot to recommend stepping up to
a toilet of that caliber *if* that's where your priorities are.
We're seeing the VacuFlush as a fairly standard item on new boats in the
moderate and higher price categories these days.