@OP- you might want to consider a cassette toilet instead of fresh water commode. This type can allow you to have a working toilet while the rest of the domestic water plumbing can remain winterized. You can have warm toilet space to do your "business" and then use bottle water for other uses.
The separate cassette reservoir means you can use RVAF or windshield washer fluid (both of do not freeze) as the flushing fluid. It is separate from the rest of the camper water system so you can mange the water/plumbing needs and uses as you deem appropriate.