I would be more concerned with a "closed" system than an " open" system. With trapped water in a "closed" system the water has nowhere to expand to... and something will crack, hence the use of RV antifreeze. With blackwater flushes, check valves and spray bars, this is an open system with the effects of gravity and no resistance to water just laying in the tube. I don't believe the amount of expansion of a drop of water here will matter.. lots of air space.
I used to just blow the lines with air on my PUP that had a shower /toilet combo and after 13 years never an issue.
2016 Flagstaff 27VRL Emerald
2014 GMC 2500HD CC 4X4 6.0L