Welll...on your boat...
I think you might be better off sticking with a manual toilet...a good one, though! For several reasons:
1. 40 gallons isn't enough fresh water to spare any for toilet flushing if you stay out longer than overnight...so you're limited to a sea water version...
2. Sea water electric toilets are NOISY, 'cuz contrary to what most people think, it's not the macerator or discharge pump that makes all the noise, it's the intake pump...and you'd also have the same sea water odor problems you'd have with a manual toilet, so you wouldn't really gain anything there.
3. Most macerating electric toilets use about 1/2 gal/flush...soo your 20 gallon tank will only hold about 40 flushes. That's max 4 days for a cruising couple between pumpouts. Otoh, if you learn how to use the "dry bowl" mode to conserve flush water, you can cut it to an avg of about a quart/flush.
So... If you still think you want a macerating electric toilet, my pick for a 27' sailboat would be the Raritan SeaEra.
Raritan Sea Era If your bowl, seat and lid are still in good shape, check out the "conversion."
But...if it were my boat, I'd strongly consider going with the Raritan PH II or PHC (same pump, but PHC is on a compact base and has a shorter handle to fit in a tighter space.
Raritan PH II You'd LOVE this toilet..just don't even THINK of putting a motor on it! Both are also available as "conversions" from the sbo.com chandlery
sbo.com PHII PHC LBA And I think you'll love the prices! too!
Both the SeaEra and the PHII/PHC can be plumbed to use the head sink drain line as the flush water intake...so we can at least solve your sea water odor problems.
That's my $.02 worth...your mileage may differ....whaddaya think?