they are 'uncommon' but you should run into them at some point. I have used this mod a few times. I usually find the trailers in parking lots or sometimes even attached to wrecked road spawns. If you enabled this mod after you played on the map already, you'll have to go to new zones where nothing has had a chance to spawn in yet. If it was added at the start off a fresh world and nothing spawned in before this mod was active, you should find something more easily.

Yup, that was the issue. What I ended up doing is deleting the chunks of the world I didn't care about, and that made the ones from the mod start to show up. I still have never seen a vanilla trailer though, hah. Thanks!