Dometic Marine has optimized their VacuFlush® holding tank technology for sailboats.  The compact SailVac 5200 is combination of vacuum pump, reservoir and holding tank and is perfect for boats 30-50’.

If you’re tired of pumping a manual marine head and the smells it can produce, check out this award winning system from Dometic Marine.  The SailVac combines a vacuum pump and waste-holding tank into one vertical, tapered unit that can squeeze up against the hull in most sailboat head compartments.  

When you combine the SailVac with a Dometic VacuFlush toilet, you use only about a pint of fresh water per flush and avoid the smells associated with standing seawater in the hoses.  With so little water used, two people can go four days on one 14 gallon tank. 
Electrical consumption is low as well with only 3 amps (12 volts DC) needed to maintain a vacuum level for the next flush. Other electric toilet systems consume 25 to 40 amps per flush.

The holding tank is made of roto-molded polyethylene resin to eliminate seams or weak points.  It has interior baffles that protect vacuum components from wastewater even at high angles of heel and the built-in ribs prevent tank distortion while building or storing vacuum.  

The system has auto shutdown when the tank is full to prevent overfilling and an optional tank-level indicator.  You can get this as a kit for $1,670 which includes the tank, harness, monitor panel with two lights, and a 4000 Series toilet.  Not bad if you're reworking plumbing on your boat.