Brian, I am the fitter for Powrtouch in my area, and the siting of the mover on twin axle vans is really down to the customer and what he wants.
Fitting in front does give some protection from road dirt and spray, but twin axle vans being that much more difficult to turn without overly scuffing the tyres, it is better to fit behind the axles.
It is easier and you have more room to play with, to raise the jockey wheel to remove some weight from the front axle.
If fitted in front, you have very little room to drop the hitch low to remove weight from the rear axle.
Removing weight not only makes turning easier and a smaller turn circle, but relieves some strain from the motors.