For those who have never pulled a travel trailer . . .. When a big truck passes you, as it goes by, it will feel as if something pulled the front of the trailer and rear of the car/truck you are driving toward the truck. It can be quite a scare for a novice...
Danger...Engineering Content: pv=mrt. All else being equal (mass and temperature on the right side, specifically), pressure MUST react inverse to velocity to maintain equality in the equation. The velocity between a rig and travel trailer (tall flat sides) increases, especially when passing the "wake" at the front of the rig. The resulting small decrease in pressure over the large surface area of the travel trailer gives it a VERY distinct and definite "tug" TOWARD the rig. While the effect is brief (as Mr. Bird says), it is quite freaky, to say the least.
Different tow vehicles react in different ways.
In a 5th wheel, you don't notice the effect at all. This is because the kingpin is directly over the axle. I'll bet you money, though, that the trailer DOES get an inch or two closer to the rig, although you can't feel it.
In a bumperpull, it literally pulls the bumper toward the rig, forcing a steering correction to "straighten" the tow vehicle.
In my brother's F350 diesel club-cab, long bed dually (long wheelbase, solid rear axle, very little sidewall give) the effect is somewhat negligible. As I previously mentioned, his wife recently drove that truck 1,500 miles through mountain passes with trailer behind.
In my truck (LT tires in back, IRS geometry in back, shorter wheelbase, less overall weight) the effect is VERY noticible. I am experienced, so I'm OK with it, but you need to be awake and alert.
As Mr. Bird mentions, if you hit your brakes hard at that very moment, there is an "angle" between the trailer and tow vehicle...therefore, the risk of a difficult-to-control maneuver (or possibly jack-knife) at that instant is distinctly increased.