Hello owenk
The model of heater you have is capable of putting out about 3.2kw of heat into the caravan when on full. But the heater is thermostatically controlled and it modulates its flame size according to the temperature of the circulation air entering at the bottom of the heater.
As Clive says when its used as a convector all the heat comes through the front grill. Its the natural effect of convection to heat the top of the caravan first leaving the floor still quite cold. As this relies on natural convection the thermostat phial can receive some residual heat reflected from inside the heater casing, and so it might begin to modulate the flame actually reducing the output power a little.
By turning on the circulation fan, and adjusting the outlet regulators, you reduce the amount of heat coming through the front of the heater, and it it tends to drag more air into the heater than by natural convection alone. This extra movement of air will keep the thermostat phial slightly cooler than by convection alone ensuring the heater operates at it best heat output for the prevailing conditions.
The blown air system redirects warm air to the lower levels of the caravan. This tends to heat the caravan up more evenly, so whilst it may not be quite as hot at head height, it is more comfortable overall.
In terms of real efficiency, using the blown air system actually makes the heater more efficient, as it increases the circulation airflow over the heat exchanger. This means it will actually drag more heat from the heater.