What kind of Jabsco toilets are they? I have had the tall conversion kits, and had endless trouble with them. I don't like the idea of them, using impellers instead of diaphragm
pumps, and Jabsco is not known for good engineering.
But the Jabsco "quiet flush" models (not the conversion kits like I have had) are completely different, and might be better. As someone correctly stated -- you should be sure you're using them correctly before you toss them overboard
I replaced one of my Jabscos with the Raritan Sea Era conversion (bought from HopCar; thanks!). I have been using it for a year without the slightest problem, and it is much quieter. So far so good -- hope this good service
One really big advantage of the Jabsco conversions is that you can pop them off in two minutes and pop on the manual pump unit in another two minutes. This is the closest I know to an easily convertible electric-to-manual marine toilet. In the case of the Jabsco conversions, this feature was useful as the electric units failed so often. Unfortunately, the Jabsco manual pump is also very carpy (sorry for the pun), so it was often a case of out of the frying pan and into the fire.
Electric toilets have such huge advantages over manual ones that I would really never go back, and if you have reliable ones, wouldn't worry too much about the manual backup.
One of the biggest problems with sanitation on board is that it is hard, with a manual toilet, to pump enough to really clean out the pipes, with the result that calcification and bad smells occur. With an electric toilet, it is easy to wash plenty of seawater through. It takes a surprising lot of pumping to get clean water all the way through the anti-siphon loops.
Electric toilets also generally macerate, which greatly reduces the risk of clogs compared to manual toilets.
I personally would never go back.
As to fresh water versus sea water -- in my experience, sea water flushing
works just fine and is mostly odor-free, as long as you pump them enough. The lines will also not calcify as long as you have pumped enough to get all the urine out (which is what causes calcification, when it reacts with sea water). You do get the blast of stench after you've been away from the boat as the sea water has rotted in the intake lines, but that goes away quickly. I don't really see much point in fresh water flushing
other than having one less through-hull, worth considering on new builds (but not so much on boats like mine which already have the extra holes).
The trick of plumbing
the sea water intake into a sink drain is a great tip, on boats where that works. That eliminates the blast of stench problem as you can flush out the intake side with fresh water (also dead simple winterization) every time you leave the boat. Won't work on boats like mine with separate gray water tanks
and pumps, however, rather than direct discharge of gray water.