Just to add to what John has said above.

It depends on what aspect of efficiency you look at, speed of process, fuel consumption, downtime, overall cost etc.

LPG trucks use more fuel however the fuel is cheaper, so economically speaking from a fuel perspective LPG is cheaper (More efficient).
LPG engines out perform Diesel trucks in acceleration and quite often top speed making them more productive in that sense for certain applications.

LPG crosses the bridge as vehicle for use both in and outdoors (generally), however AC electric trucks are also in that same position....but then it comes down to fueling options.

LPG although easily available in Qatar would not be my first choice due to the extreme heat in the region, where diesel generally has no issues. Maintenance practices also play a part and LPG engines can be a lot more sensitive to quality of fuel and servicing compared to diesel. The ambient temperatures in the Middle East as a whole mean that cracking of the LPG is more likely and so you could expect more regular servicing of an LPG truck.

LPG trucks also come to certain markets (Generally not in Europe) with dual fuel option, which provides the flexibility of being able to switch between gasoline and LPG. Which may mean only stopping to fill up at the end of a shift, and also reducing downtime because the bottle ran out - Away from the where the new bottles are.

As John said LPG has the advantage of fuel delivery and storage, however if you have a fleet of vehicles used off road i.e. in a port etc. then Diesel makes far more sense.

Diesel trucks are generally not suitable for applications where exhaust particles can contaminate your product i.e. paper mills, food processing and handling final products for consumers. However a catalytic converter and particle filter fix that....but initial purchase price then goes up as do the long term running costs.

For a long time to come Diesel will be the fuel of choice in the Middle East and Africa.