Where ya been sunshine???
Al has the ticket with that portable MR Heater, That should solve your freeze up problems
you might also get some monitor sensors with alarms so while in the house you can tell what the internal temp of the truck is.
Back in the early 80's when I ran the TMs in Anch. Alaska
We had all trks that had cleaning units in them sprayed with foam for insulation. cube vans we sprayed 2 inches to the floor from underneath. walls got about 1 1/2. For when a rig had to be left out or was we had multiple space heaters going. Never had a rig freeze up that was intensionaly left out in freezing temps.
The first TM I saw was a Steamway PowerMatic. They had mounted a propane catalic heater on one wall with a thermostat. They called this truck the bomb. Ran on gas, burner fired on kerosene and heated with gas. The poor truck was all tanks, 31 gal gas,24 gal kero and 15 gal propane underneath and the 105 SteamWay waste on top. It was sprayed with foam on the side walls, the ceiling,and on top the floor with plywood on top of that. Comming from the fishing industry and seeing what happened to boats with sprayed foam that caught fire this thing scared me especialy the first time I was in the rear working on a clapsed hose on the live reel and the heater lit off and caught my jacket on fire. I had my back to the heater and my jacket just got to close, Yes I was wearing the flippen jacket it was 20 below..