CB Radio or the citizen band radio is one of several personal radio services defined by the FCC's Part 95 rules. It is intended to be a two-way voice communication service for use in personal and business activities of the general public, and has a reliable communications range of several miles. This link on wikipedia tells us about this.
It also states that CB Radio is most frequently used by long-haul
truck drivers for everything from relaying information regarding road
conditions, the location of speed traps and other travel information,
to basic socializing and friendly chatter.
So it is safe to assume that if one is using the CB radio to chat with someone they are more likely to be on the same route, though they may be separated by a distance of about 50 miles. Still, we can assume that if you are talking to someone on CB radio, they are relatively near by. So Fuller takes a shot in the dark. He is not sure whether Rusty Nail is really close or whether he will accept his offer to meet at the motel which might be a few miles away. He is just fooling around and he just takes a shot to see what happens. Coincidentally Rusty nails accepts his offer because he must have been around.