I run a re-circulating pump to keep the water moving through the system, insulated my trailer and heat it with wall mounted electric heaters. I have redundant systems in case one fails over night. This past winter really tested my systems, dang it was cold.
I was thinking along the lines of what flat-workers (word I learned here) do for income when its too cold to put water on the sidewalk? I've heard that water stops being a liquid and turns into a hard slick substance in some far away places.
We have a special anti freeze system built onto our washer. We also store them in climate controlled warehouses/garages when not working. Russ taught me a good trick as well. You can buy a grill cover and put it over your unit - if you find one big enough - and then keep a heating pad bungeed around your pump....If you have a generator built into your washer just use that as your electric supply -
But to answer more of your question - we dont pressure wash much in the winter, we plow!