The trouble with diesel heaters is that they use a fair amount of current to get started.
To keep a van above freezing (my electric froststat kicks in a 2 degrees and off at 4 degrees) would mean that heater firing up numerous times through the night. Battery drain will be a major concern. If that cleaner hasn't off road parking, how will he bench charge his leisure battery?
There could be the other option of fitting a very expensive lithium-ion battery that will accept very fast charging with temperatures above freezing. But this could mean some alternator modifications plus the cost of the heater.
A propex gas heater requires a minimal amount of current to start it but probably uses a similar amount as a diesel heater with the fan running.
One of the local lads has on street parking only. He drains his pumps down at night during the winter and brings his hose reels inside. But we don't get the cold on the North East coast some experience futher inland.
Another further north insulated his tank and lagged the pipe from his tank to the pumps. He added 12v heat tracing cable between the pipe and lagging. When he started the van in the morning he switched this heat tracing cable on and it warmed the pipe enough to melt an ice in the pipe. He also drains his pumps down and at the same time pumps the water out of the pipe from his pump to his van port.
On a side note, I wonder if its a viable option to look into all these charging points for electric vehicles that are now becoming more widely available and make use of them.