Troy
You didn't say if you are closing it up for the season or intend to operate all winter.
If you are closing it up you can run NON TOXIC antifreeze ( the pink stuff) into the system by emptying your drop tank and adding antifreeze . Don't use auto antifreeze as it is toxic an youwillhve to dispose of it in the spring in compliance with EPA regulations. Run the machine until you have pink coming out of the gun and the pump, heater, hoses and gun should be ok. Don't forget to blow out your chem lines with air and empty your chem tanks or put them in a warm place.
Any water anywhere will freeze an cost you big time.
I blow out all of my lines, take off my pumps and empty all my chem tanks and don't put any anti freeze in the system as I completely empty it of water.
If you are going to stay open you need to get enough pink stuff to run through your system nightly and then pump it out with water in the morning and go to work.
If you are putting a heater in you truck use a "radiator heater". This looks like an old fashioned radiator and is full of oil. There is no exposed heating elements or flame and they are pretty safe.
Use common sense and keep the heater away from any flamable or combustible materials.