Just to clarify, are you talking about Tony's schafter?
They only seem to appear when the player is driving one himself causing others to spawn, I've never encountered one randomly anywhere in the city so I guess they are just not programmed to spawn, You can check the vehicles wiki page and they should tell you where they are meant to spawn, but even I've tried that with no luck
So all you can really do is replay missions and keep tony's schafter afterwards, save it and use it to spawn others on the road.
It's a weird thing R* did for the dlc's with many vehicles only appearing once in missions
and vanishing afterwards
Schater 2nd gen
Anyway hope this helps