I decide on the Thetford Syle II with a ceramic bowl and water saver sprayer.
Thetford Style II parts can be found at various RV stores, Thetford, RV dealers, Camping World, eBay, Walmart and Amazon.
RV toilets are usually capable of using as little as 1 pint / 0.4 liter of water. Whereas most tank flush toilets (water saving type) use at least 1.3 gallons of water per flush.
Kohler who also manufactures small footprint toilets using smaller tanks that are not separate from the toilet bowl. I remember seeing one with a 1 GPF rating.
There is a tank type toilet for sale at Home depot for $150.00 that is rated at 0.8 GPF.