Firstly, did the heater work before or is this a caravan that is new to you? Is there a master switch for the heater's mains or a spur fuse? If you had asked this in winter I would have suggested you switched on the heater without the fan and when heat was coming out at the top switch the fan on. However as it is summer this might not be helpful. The heater, I think is really gas and the electric elements are in a position that doesn't optimise their output.