This would be a 'Mk2' Fanmaster from 1998 until 2000 on the back of a 2000/4000/5500 gas fire with a slide control on the fire top.
While Craig is not wrong this system is best put on 'full bore' both gas and mains for the first hour to an hour to get best results.
The reason which also to a degree applies to the Mk1, is the heating system needs to warm the whole structure of the van from cold which requires a lot of heat before coming up to an acceptable 'living' temperature. The difference is the Mk1's mains side will cut out fairly quickly leaving the gas alone, while the Mk2 version will continue to work together. This little fact is why we have/had the 4000 and the 5500 which are in fact one in the same fire!.
The internal structure of a van has an enormous amount of cold surfaces which take an equally enormous amount of heat to come up to an acceptable room temperature..this from a relatively small heat source.
Once the structure is warmed through, it then acts as a reservoir which will give off heat to the air if it's temperature is reduced by say a door being opened. Until then it continues to suck heat from the air and further reduces it's temperature.
This assumes there are no problems with the system and one possible is there are two separate elements, if one fails this could be a reason why you have the problem but the fan is controlled by the heat given off and if solely using electric heating I would doubt it.