Hi,I have had this problem for years what i find helps is to lag your water tanks .
Even an old blanket or too will do this also cuts down on the condensation on the inside of the roof which cant be a bad thing it wont stop your outlet pipe from freezing up if you can remove these at night when your done befor the next day I would sugest that also dont leave your power washer in your vehicle at night as the hose on that can freeze.
Having had the motor freeze on a jet wash in the past this all makes sense it can happen and they just cook in front of you .
If it gets to the point where my pipes are frozen and the tank has ice its not a good day to wash cars .
I have come tao accept there are some days that you just cant wash cars also you want to avoid spraying water in zero temps we need to consider some one slipping and claiming damages .
hope this helps.