[skyking]

we are saying quite a bit of difference. The template works why? Does it reduce the sail area as a trailer moves forward?
No.
It reduces the low pressure that happens at separation on a square ended vehicle, at the back end! Look at all of Phil's Hutcho charts. The front end, or front side does not matter in the wind tunnel.
This happens just exactly the same in a crosswind. Your Airstream is a prime example of how to do it right.