Sea Era
I have a Mark I C36 and we did this a few years ago after I had to rebuild the original head while rolling in a choppy anchorage...twice! Being on the Great Lakes (as you appear to be) we didn't care about overboard discharge at all, but cared about maximizing our holding tank capacity.We went with the Raritan SeaEra electric toilet with the freshwater flush option. You can get one from Boater's World for less tha $500 for the whole toilet. It uses pressurized fresh water from your tanks to flush, but our C36 tankage is such that you'll fill the holding tank before you drain the water tank. The nice part is that the freshwater flush lets you use minimal water to flush so your tank fills slower. We (2 adults, 2 kids) can go a good 4 days using the head exclusively without filling up the 18-gallon holding tank. The cool thing is you just push the button to flush it. Once in a while a little extra push is needed to get a stiff contribution down into the bottom of the bowl, but its FAR less cantankerous than your typical marine head.When we sailed out of the lakes and into salt water we found another bonus: No stinky salt water in the head!