You have to see that your curved front TT is essentially the same as the leading edge of an Aircraft Wing.
The distance the air goes over the top is different than the distance on the underside. This creates LIFT.
What has as much effect on this as the trailer is the tow vehicle and its aerodynamic properties..ie how much air it lets under the trailer.
This can increase or decrease the effect.
And YES this effect is real. A cessna 150 can take off at 50 mph in the right wind. Your TT could experience enough lift to affect the towing characteristics...especially if it is not loaded correctly with the proper tongue weight.
Thats why these new round fronted trailers are touted as being Aerodynamic.
This idea of the salesmans may or may not be your true problem only testing could tell that. But essentially what he said is true.
