Good throught, but I drained all tanks prior to this so it wasn’t from a winter freeze. Plus the tanks had anti-freeze in them over the winter, which I had drained out earlier in the day.
What I did yesterday was fill the fresh water tank with water bleach mixture and ran it through all the water lines, purging the lines of anti-freeze and filled the HW tank with water-bleach mixture.
Then I emptied the black and gray tanks to empty them of winter anti-freeze and the water-bleach mixture put in them from the sanitizing process.
So the black and gray tanks were empty when I blew out the water lines. The water that was in the black tank and froze was only from blowing out the water lines.
The other thought I had was I use a small compressor to blow air through the city water intake to blow out the lines. Could that forced air add some additional cooling affect to the water?
Mary & Tom (aka Hutch)
2015 Jayco Eagle Premier 361REQS
2014 GMC Sierra 3500HD 4x4 CC D/A
Sunlines: 2005 Solaris T-280SR; 1999 Solaris T-2670; 1998 Saturn T-24A