As Dave said, its not really a problem to dip below freezing for a few hours.
The black tank will be fine even if it did freeze. The problem is the expansion of frozen water and so a few gallons in a large tank won't harm anything within the tank.
The lines are where the most problems can occur, and even then Pex has a tendency to flex somewhat. If it helps alleviate concerns, open the low drain valves and the faucets to drain most of the water in the lines. Or as you said, blow them out.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD