I have the Hallmark Milner with the eco Truma Combi plus and have used it in below freezing temps, but not as cold as described here. It is so quiet and efficient. I have gone for 3 nights using it at ~ 60 degrees and only using 25% of my battery. The ducting runs next to the fw tank and under the sink. There is a vent to the lithium battery so it stays at room temp and performs exeptionally. I do crack the cabinet under the sink for good measure, but don't think it's needed. If worried about the water in the combi as along as the heater is on you'll be fine. However if the heater will be off, then it's best to drain the water from the combi. Here's a writeup I just put together on the Milner: https://www.surfands...ark-Milner-page
We are about to order our Hallmark - new - and the Truma Combi Plus is, by all accounts, going to be a game changer.
It is super efficient, and also super quiet - and it is ducted so the heat goes not only into the Hallmark main compartment, but they duct the cabinets so the batteries dont freeze, and the 30 gallon water tank does not freeze... (if you havent winterized.... depending if it only dips down into freezing at night - which is when you'd be running the hot air....
so we are excited to do winter camping with it.
If it was going to get REALLY cold... then i'd blow out the water out of the hot water tank, but even then the Truma Combi Eco Plus can still make hot air on either propane or 120v... so we can try camp but still be very comfortable..... just no hot showers 'till it gets a little warmer and we put water back into the system.
I *hate* the super loud old fashioned RV furnaces so for me the Truma Combi eco Plus is worth the added expense.
Game changer.
J.