It's a poly tank with the inlet on the top. It looks like it's threaded, though I didn't measure it.
A fairly easy job to install a new 1. 5" inlet ftting in the tank using a Uniseal
UNISEAL
I want to moving to vacuflush for a few reasons.
1) Holding tank capacity: The electric flush uses raw water and far too much of it. I have a 30 gallon holding tank that I need to empty more frequently than I would like.
2) Odor: The electric flush uses raw water. I only use the boat on weekends. The saltwater leaves an odor in the head after baking in the heat for a week.
3) Noise: The electric flush is loud
You're assuming that all electric macerating toilets are alike. Most are available in both sea water and pressurized flush versions. While all sea water toilets are noisier than those the use fresh water, some are a lot noisier than others. The Raritan Marine Elegance
Raritan Marine_Elegance Promo.pdf --the top rated electric toilet--is VERY quiet...in fact the quietest. It's designed to use pressurized fresh water. The optional "Smart Flush" panel provides 4 flush options that will actually allow you to use LESS water than the VacuFlush actually needs to prevent problems. And there are several others you might like.
I estimate that at least 75% of the toilet problems I've seen in the 30 years I've been a specialist in marine sanitation are problems with VacuFlush, and that at least 9o% of 'em are caused by owners putting too little water through 'em. It's the reason I wrote a piece I call "VacuFlush 101" that describes how it works (an amazing number of owners think they know, but actually don't) and how much water it actually needs. I'll be glad to send it to you if you'll send me a PM that includes your email address (no way to attach anything to a PM). An earlier poster said it's 1980s technology...it's even older. It's changed so little except cosmetically since Mansfield Plumbing introduced it in 1978 (they spun off their entire marine toilet division to SeaLand Technology in 1984) that the same troubleshooting guide still works for all years, all models.
And btw...VacuFlush was the toilet on my last two boats and I was a distributor/dealer for nearly 10 years, so I'm intimately acquainted with it. If I were looking at used boats that already has one installed, it wouldn't be a deal breaker, but macerating electric toilet technology has changed so much in the last 20 years that I wouldn't spend my own money for another one.
Send me a PM...and I'll not only send you "VF 101" but I'll be glad to answer all your questions about it and every other toilet.
--Peggie