While I always drain the holding tanks, there is a bit of wayter left inmost, so I go one step further and use the pink to add a cup or so in each holind tank and certainly in every drain trap!
\What I have found is that it seems the taniks are empty but if I just close the valve there is often some still draining which can collect att eh end of the pipe at the valve, so adding some pink makes that less likely to ever freeze.
The traps are one which is easy to miss----like the one in the shower? They are meant to hold water to block odors but they also do a great jjob of holding water to freeze and break!
Also look at the little plastic filter at the water pump? If it is set with the bottom up, it drains but if it has been installed bottom down, the water is often left to freeze!