[aerohead]

On the semi's,the trailer bottom is almost 4-ft off the road surface and this gap cannot support a transverse pressure gradient,as it just bleeds into the physical void.
The F-150 is lower,and side-to-side,does create a boundary in which the air is accelerated between the two of your vehicles.
You experience the Bernoulli effect,as the static pressure is converted to a partial vacuum by the displacement.