The Saunil Mobile Catapults spawn at the respectable locations, then move towards Trina Fort, where they will stop and shoot.

3 can be found west by the road leading to Trina Fort.
1 can be found east by the road leading to Trina Fort, almost at the gate entrance.
1 can be found east of the road leading to Trina Fort, near the main road.

Hopefully this information helps a bit.