Our diesel heater works fine. Access is via the camper control module under aux heating. It is not the parking heater which is for frosty days and clears the windscreen.
We set it to 4 the other night when it was 5 degrees outside and it kicked in a couple of times during the night. It may have been too warm recently for it to work on anything other than maximum for you. I am not sure what temperatures the heating kicks in for the numbered settings.
Remember that instant heating is for warming the van if you are stopping for lunch or something and generally requires the van to be driven afterwards. Continuous heating is the option if you are in camping mode and it will kick in when the temperature drops in the van. The warm air vents out under the kitchen block at floor level.