GordonThree - you are a tough dude, and obviously a talented RV'er to upgrade your camper to handle -15. That's when things start getting a bit chilly up north.
As far as "Dry" camping - well I suppose it's semantics. Since I never camp with hookups anyway - call it boondocking - to me not having 40 gallons of water on board is a bit "drier" than normal. Call it "Semi-Dry" camping.
I don't leave the black tank dump valve open, ugh, that would be a bit nasty. I use about the same amount of flush water from my water jug as is used during "normal' flushing.
For the temps that I'll see - in the 20's - using a mixture of pink antifreeze will work OK, not perfect, but OK.
And, since we live in Montana, we take showers every other full moon. We discovered some time ago that there are other ways of washing instead of showering.
Bob