Its a simple trophie if you have allready few outposts around the map and the RPG unlocked. The game will warm you (at top left of screen) when a convoy appear. Then check his position on world map, do a fast travel to the nearest outpost and stuff in your RPG. Check again the map for choosing a "meet point". Now wait near this point, vehicule parked on the road. The first shot is for the first convoy's vehicule, 1 or 2 shots for the two last vehicules :)