Campsite warp or ride to Seth's next location and you'll find him back up to his old tricks still. When the cut-scene finishes, take the reins of the cart and follow the familiar yellow line showing your route to take.
Not long after bypassing Armadillo (if you try and drive through Armadillo and get stopped by the coppers, you'll fail the mission), a group of persistent treasure hunters on horses will appear in the distance.
Attack them when you're on a straight stretch of road, and do your best to nail them before they get too close and start lobbing annoying Molotov Cocktails at your wagon! They'll be on your case for a good few minutes, so keep your rifle at the ready at all times.
Once they're all taken care of, you'll need to follow the yellow marker to Tumbleweed, where you'll drop Seth off at a cemetery and complete this mission.