This may not work for you, but there is a simpler solution, as I see it, for short trips. Last winter, I took DS on three different overnighters when the temps were below freezing (mid 20s on one trip). I left the water system winterized. I took cooking/drinking water in a 7 gallon aquatainer. If I needed to pour any water down the drain, I poured it into the toilet, and I used the water I took with me to pour into the toilet for flushing. I used the toilet for #1 only. When I got home, I drained the blackwater tank, then poured more antifreeze into it.
I used paper plates for meals, and just placed any dirty dishes in the sink and washed them when we got home.
I don't know at what point with temps the blackwater tank would become a problem. Mid-teens might not work the way I did it. You would need to keep adding more antifreeze, I'm sure.
To me, the extra work required to use your on-board water system would not be worth it for just a weekend trip. I would find a way to get by with the water system still winterized. Our local state park has a heated bath house to make this easier. This may or may not work for you.
