Or home heating oil with a drop of cooking oil in it,much cheaper.You'll need to look at the vehicle handbook and see the make & model of the actual heater, eberspacher for example.
Then take a trawl around the net, you should be able to dig out the fuel consumption data.
It'll be listed as X litres per hour at full output, x/y at half output etc etc
If you have a separate heater fuel tank, you can use red diesel available from most marinas.
James
Curiosity only - we wilded in what we thought was the middle of nowhere ..certainly wasn't on the POIs, only to be joined by a couple of Aussies in a rented mh. They were prepared to be cold because their diesel heating system smelt so bad ..... is that normal ( the heating/diesel bit, not the Aussie part)