I’ve been in freezing weather overnight with temps getting above freezing during the day in my Class C. I have a slide and run an extension cord to the pedestal to run my 1500w oil filled electric heater and we keep warm. Propane heater tends to come on early in the morning. Holding tanks are fine in those conditions.
However, being below freezing night and day is a different story. Holding tank contents in a Class C will likely freeze unless you add anti-freeze to them. I have heated tanks and would not trust the heaters to keep up. Plus the coach is not insulated well enough for that environment. Everything I’ve read says “don’t do it, it’s not worth the risk”. I tend to agree and personally would not stay in below freezing weather for more than a day. That’s my engineer based assessment. You may be more risk tolerant than me.
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