Here in central FL, at freezing temps, I just usually dump a gallon of pink stuff in each tank, valves closed. One night a neighbor suggested just leaving the gray valve open and trickle water. OK. At 4 a.m. DH woke me up,
something wrong with toilet... burping back at him. Checked gauge monitor real fast, yup... on red. Went outside and pulled valve. Turned off water into "GRAY" tank. Called mfg that morning... we had the only model of that brand which the bathroom sink drained into the black tank. Don't drip water anymore!