There was some considerable research looking at fuel efficency of large lorries done in the 1970s, I susspect as a result of the earlier fuel shortages.
Large vehicles that had very sharp corners to the tractor and the payload area could have their fuel efficacy improved by several percent by adding rounded mouldings to the leading edges.
This apparently smoothed out the air flow around the leading edges, making the vehicle slightly more aerodynamic. The reduction in the turbulence caused by the sharp corners, meant that the air flow around the side of the vehicles was less turbulent and will approach laminar conditions on a long artic. It also means that the air flow from the trailing edge would settle quicker as the vehicle passed.
Each design of vehicle will have its on characteristics which will change with speed, but as Lutz points out the two events of the leading edge and trailing edge will occur more rapidly with a short vehicle.
In general, the easier a vehicle can slip through the air the smaller disturbance it will cause to other road users.
Theoretically, the worst case scenario for a caravan is to interact with a vehicle of similar size, If we assume the bow wave with tend to push adjacent vehicles away, and the trailing vortex will tend to suck adjacent vehicles towards it then the following sequence of events will occur.
Transit sized van begins to over take. Initially the bow wave from the van will tend to push the rear of the caravan to wards the near side. This slightly turns the caravan and its track will begin to drift towards the off side.
As the vans bow wave draws level with the caravan axle, there is no net turning force, but a general shove towards the near side.
When the vans bow wave moves forward of the axle, the front of the caravan begins to experience a turning force towards the near side, but at the same time, the vans Tail vortex is also pulling the rear of the caravan towards the offside. These two forces are additive, and considerable. The caravan will try to track towards the near side.
As the van passes, the tail vortex will eventually begin to suck the nose of the caravan back towards the offside.
If the van were longer or a full size lorry, the bow wave and tail vortex forces would not occur in phase with the caravan and thus will have less of marked effect.