Most likely it's a bluetooth wireless signal and during the day there are a lot more bluetooth devices around that are causing interference. But that's just a guess. It could be wifi. I couldn't determine if it was bluetooth or wifi from the ad. In either case, range extenders are available on Amazon.
I would start by asking the system manufacturer, Intellihaul what could be the problem and if they have a solution.
You might be able to move the receiver part that is inside the truck cab closer to the rear, thereby reducing the distance from the camera's transmitter to the monitor's reciever. Then fro the receiver to the monitor is probably coax cable of some sort that you could just extend.
For example; mount the receiver in your 3rd brake light with a coax from there to the monitor. You would be reducing the distance between TX and RX and still getting the video signal to the monitor.