Raritan Sea Era Practical Sailor rated it "best budget buy" in their Feb issue this year. It's available in both sea water and pressurized fresh water versions (I strongly recommend going with the fresh water version)...both versions are available as complete toilets or as a "conversion" that allows you to save money by re-using your existing bowl, seat and lid.
Raritan Sea Era Conversion Both versions can be installed to put discharge on either side or out the rear.
I would spend the extra money for the optional 3-way flush button (ONLY available for pressurized fresh water version) that provides a choice of only bringing water in---which alllows you to add water to the bowl ahead of use, dry bowl, or a combination (same as single button flush)...it'll really cut down on the amount of flush water needed. I don't think Raritan mentions that option with the SeaEra...they do for the Elegance.
Electric toilets should be on their own dedicated circuit shared by nothing else that can pull power away from it, so you'll need to run a new circuit and install a new breaker for it. That's the only part of the installation that's really any more difficult than swapping out a manual toilet for another manual toilet. READ THE INSTALLATION INSTRUCTIONS carefully!!