Since I always carry an empty 1 gallon milk jug, after I've drained the blackwater tank, I close the valve and pour freshwater in the toilet... maybe a couple of gallons, then drain again. That's not only rinses the tank, but the hose as well. Having a few cups of clean water behind the valve is not problematic, except during a freeze.
Ten Forward - 2014 Escape 5.0 TA
Double Yolk - 1988 16' Scamp Deluxe