With your UHF on Channel 40, its as simple as making the following conversation when engaging in a passing manouvre.
YOU, "Red truck behind the Traveller caravan, are you on channel"
HIM, "Yes Mate"
YOU, "Give us the drum when your want to come around"
HIM, "Having a look at you now"
YOU, "Go when you're safe"
When his cab is going past your vehicle, you can back off a little if it is safe to do so, bearing in mind there may be other vehicles behind you.
If there is no distant oncoming traffic, maintain your speed.
Once he has passed, you can finish by saying, "You're clear''
Hope this is of some help.