Jason,
It would take hours to unload/reload a packed Transit - "old-timer" or not.
The van is ply-lined to the waste, insulated with felt-backed carpet to the roof and the roof is insulated with polystyrene plus the insulated, metallised underlay used for laminated flooring.
At night, in cold weather I use thermostatically-controlled Frost-Fighter heaters.
Be aware that in extreme cold, the van contents (liquid chemicals included) are also at risk from frost-damage, hence the emphasis on insulation.
The really clever part of the scheme is to remember to disconnect the power cable in the mornings before driving off to the first job
