Hi Steve,
I agree in that the fresh water and hot water tanks are difficult to drain out completely. It sounds like you did your best and that is that.
If our PC was stored outside in sub-freezing temperatures, I too would be tempted to keep it warm inside....to a point, not only for freeze-protection, but also to remove dampness. What comes to mind is some of what you had already mentioned.
- Set the Ford cab HVAC to "recirculate" to prevent outside air from entering through the dash vents.
- Cover the windshield with a well fitting thermal bubble/foil sunshade.
- Come up with something similar for the van door windows.
- Drape something like a warm heavy sleeping bag unzipped wide open, across the two front seats and tuck it in and around to eliminate as much cold cab air from getting into the house, keeping in-mind that cold air is heavy.
- Seal all roof vents with Styrofoam blocks.
- Open all cabinet doors & drawers, and also the bathroom & shower doors.
- Draw all shades as they offer some limited insulation.
- Place a tiny ceramic electric heater inside on the floor, placed away from anything that might fall on it like the sleeping bag or Styrofoam. Set it on something non-flammable, like a section of plywood with sheet metal laying on top.
- Set the heater on the lowest setting as possible.
- Place a thermometer inside to monitor the inside temperature to help determine the right heat setting.
