Hi Gijs,
I marked the parts of this document that are wrong (the engine definition) or obsolete after my last patch to your code in CVS. Please review these changes in the document.
A suggestion for your towtruck: What do you think about relocating the object, so that the bottom of the wheels are at Z=0 and X=0 where the nose wheel is supposed to be? (X and Z in FlightGear coordinates where Z points upwards and X points back) This makes it easier to place the truck by directly specifying the coordinates of the nose wheel as the position for the truck.
Greetings, Torsten