I assume its similar to the carver system of hot air circulation from the room heater. I could be the fan is not producing enough oomph to circulate hot air to the rear so front being shortest run no doubt takes most of the heat and just warm air to the rear.
Check the hot air pipe feeding the rear has not come off the fan housing
Also check the controls for fan speed if it has a variable speed control.
First check with local caravan shop you can get spare tube and parts for your unit.
Check under the van, if the pipe to the rear goes through the flooring, there is no damage if their is then obvious problem with heat loss. If no damage try the following insulation blocks as used in the building trade the one like thick polystyrene cut out a channel to fit the pipe and fix in position over the pip using flexible plastic strips screwed to the floor that should prevent heat loss in cold weather.
bob, February 2013
