After being "flashed" over by another truck, it is only good manners to say thank you! This is done by flashing the trailer lights.
You can do so by flipping the switch on the dash or by putting your four ways on and letting them blink 2 or 3 times. You can even pick up the CB mic and say it to the person. You can say "thank you" in this fashion in other situations, also – when another truck stayed back so you could complete a turn, etc.