If I have a large water tank (2000L) fitted to a moving vehicle and I have an elbow high up on the end wall to act as an air vent and it becomes submerged during inertial surge of the water, will the pressure wave in the water create a forced overflow condition pushing water out of the pipe even if quite tall or, will the head of the water just reflect atmospheric like a static pitot tube?

Yes It looks like it would. Turn tank around so that vent is next to cab.