Let's get down to basics...
Most important, is/are the battery(s) fully charged and in good condition?
How long do you leave it with the motor running?
Can you hear the fuel pump ticking after a while with the motor running? The pump is below floor level so is probably best heard from outside left side of vehicle.
Does it stop on its own after a while or only if you switch it off?
Is there a display on the control panel and if so does it show any fault numbers?
I'm not totally familiar with the version of Eberspacher on your vehicle but I guess it's a version of a D2L or D2LC dependant on age. They are all very similar except for the actual control panel.
It's worth checking for blockages in the heater exhaust pipe and combustion air input pipe, also if it has a separate fuel filter is it OK. The fuel take-off from the engine fuel system is accessed via a removable plate under the floor covering below the left front seat.