I would have a couple of concerns. It is very difficult to be sure all of the water will drain from pipes by simple gravity. There are almost certainly going to be low spots that won't drain. You should have the pipe blown out with compressed air, but do not blow air into the toilets. P traps should be protected from freezing as well, but they need to remain full of liquid to block sewer gases and vermin. Some folks use cooking oil, other like RV anti-freeze. (NOT automotive) The toilet bowls and tanks need to be drained, and the toilet traps protected with oil or anti-freeze.