Been having a hell of time trying to get convoys to appear post game (Prince of Thieves cheevo). I came across this by accident, if its been said already then I've missed it and apologize.
On the Frontier map in the southern fast travel (bottom middel of map), RUN (do not use horse) East past the general store and follow the road that travels slightly North East. Stay on the main road while running until you come to a Y-intersection. From this intersection continue East (right) from Y-intersection running until you get to the farm. At the farm turn left past the first tree (North) so your jumping the fence and running through the field. Continue North ever so slightly North West to avoid a couple of spruce trees. Continue running till you get to the road and rock hill at this point prior to hitting the hill a convoy will appear.
I've tried this with a horse and went 0 - 3 attempts. Tried this method running and went 7 - 7 of 1x convoy with about 7 guards. If this works great, it did help me get prince of thieves cheevo.