Prerequisites: You must have finished Chapter 6 and reached Epilogue Part I. This is important since the majority of the treasure hunt takes place south-west of Blackwater, in New Austin which is inaccessible earlier.
Full Video Guide
The Elemental Trail Map 1
In order to find the first map and start this hunt, you need to head to the far west of New Austin near Sea of Coronado.
Once you've reached this area, open up your map and you will notice a black cross on the map, which is an indication of a corpse. Go to the corpse and you will notice it hanging by a rope attached to the foot. You need to shoot the rope and the body will fall off. Loot the body in order to get the first map.
The Elemental Trail Map 2
The first map has a drawing of a chimney and a pair of cliffs in the background along with some water. You can find this chimney in a burned out house east of Lake Don Julio, south of Armadillo. Once you reach the house, search the chimney to retrieve the second map.
The Elemental Trail Map 3
The second map depicting a rudimentary aqueduct refers to Benedict Point, located to the South of Tumbleweed.
The map is tucked into a gap between two planks of the unused aqueduct.
The third map contains drawings of a cross and a church tower. One refers to a church while the other one is a reference to a grave. The church image has an arrow pointing to the left and the grave image has an arrow pointing down.
This is a reference to the Coot's Chapel, which is located south-east of Armadillo. Once you reach the chapel, go to the cemetery which is located to the left of the church. In the front row in front of a tree, you will find a grave with a wooden cross. Examine the rock next to it in order to retrieve your treasure.