a truck stop parking lot is very dark, most drivers turn their headlights off when entering.
If you are parked in the lot, sitting in the driver's seat in your truck, 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.