Im sure that the organisation concerned will probably go through this but:-
Perhaps you could ask about:-
1) How a fifth wheel coupling works (the device which hooks up the trailer to the tractor unit on a articulated rig)
2) The function of the coloured hose lines you find on the back of tractor units hooked up to trailers
3) How to raise an articulated trailer by hand if power to the vehicle or unit has failed
4) How cabs on trucks can be tilted to give acces sto engine compartment (will differ dependant on make and manufacture but generally they are all similar)
5) Common Location of airbags on trucks
6) Discuss access problems associated with height of driver in cab (think about hauling up heavy cutting equipment to cab level if you ever had to extricate a driver or passenger out)
7) How braking systems work
How to remove locking pins from fifth wheel couplings
9) ADR / Tremcards etc etc
10) Weights and lengths of LGV vehicles
I could go on but maybe that will do for starters Andy?