When Gilmore shows Aileen the distance to the waterhole, there is a clear stretch of sand between the mosque and the waterhole. When Aileen later goes towards it, there are rocks and building debris visible.
Gilmore and his cohorts stop the lorry on a hill overlooking the mosque, presumably because they were concerned about the ground surface. Later though, the plane is able to land on the ground just outside the mosque indicating that the surface would have supported the lorry. If the lorry had been parked closer, then the Bedouins would not have been able to steal it and perhaps Gilmore and Donovan would have been able to escape.