Had the same expierence however the temperture was a bit lower (-6) than you had.
Odds are it is not running out the low point drain. Said drain is often a hose which goes through the bottom of the compartment, and it's leaking inside the compartment and running out the hole the hose goes through, Without investigation it would be impossible to tell the difference.
Odds are one of the hoses in the water compartment, behind the panel with the connections on it, has burst or cracked, In my case it was the inlet to diverter valve line. The easiest one on the whole rig to fix I would not count on that kind of luck
After some well-getting to know my plumbing time this morning and afternoon, I found two problems. One is a crack in the kitchen sink drain which has been but a small drip that has thoroughly saturated the floor. I hope it dries well enough for not having to replace. The other problem was the direct cause of the low point drain spewing this morning. After cutting through the insulation to get into the underside of the RV, I found an elbow valve that had somehow been opened. I closed it and that fixed the problem. 'Tis a little mysterious as to how the valve was opened. We've been living in the RV for 2 months and this is the first plumbing issue we've had. We're good to go now. I've got the valve secured with some trusty duck tape until a new valve gets here. I'm replacing it to be safe than sorry.
Not sure what an elbow valve is but it was common on my boat that when the lines would freeze it would push the 90 degree elbows apart. Solved the prolems by replacing the copper tubing with high quality reinforced flexible line using no 90 degree elbows.
Sounds a lot like what I just did with one of my water lines Weewum.
And yesterday and today I did something I have NEVER done with this motor home before.
First, (This I've done before) Since I've been working on one piece of line I have been running off stored water and the tank ran dry so I went to the water bay to add water... Bay was dry. I soaked it well but it was dry, never been dry before
Let it dry out and when I left to do some shopping, lunch and take the wife shopping (different stores) I put pressure to the line that has been giving me problems... Came back to a completely dry bay and no water spot on the pavement below
Finally I put city pressure (Via a regulator) to the affected line. I am expecting to see a dry bay and ground when next I look
As I said, I've never seen that before
Replaced the problem line with a quality flexible line same as you did