A couple of questions:
1. How cold is the freezing weather and what is the expected wind speed hitting the coach?
2. What kind of pass-through charger does your coach have - BIRD/Big Boy?
3. What does not work on the HydroHot - diesel burner, electric element, or both, or is something else not working?
My comments to:
1. A few degrees below freezing for a few hours, like overnight, especially without wind, is not a problem. A few space heaters would do the trick. Otherwise, 5 to 10+ degrees below freezing for more than overnight is a problem.
2. Our Big Boy was acting up a few years ago. My nephew and I removed it, opened it up and cleaned it thoroughly, put new di-electric grease in it, and it has worked well since. Last year, I found the two 12v engine batteries low. We reset the BIRD by turning it OFF and back ON to fix that problem. I was told BIRD modules are notoriously problematic.
3. Our AquaHot has been a problem over the years too. I have replaced the igniter module a couple of times, the controller a couple of times, the electric element once, etc. Three years ago, the technician replaced the controller... and that did not fix the problem... which I learned after leaving the area. Last year, after two years of not igniting the diesel burner, the igniter module was replaced. The AquaHot has since worked flawlessly.