More precisely, this quest can and should be played after Declaration of War main-quest.
After you successfully finish Dust to Dust, Dave will move to Coernix-Station Alstor outpost, located in the northern Duscae.
Look for him in front of a shack behind your trailer, and see what he has to say.
As usual, Dave wants you to retrieve a Dog Tag for him, but this time, you have to travel west.
Objective: Look for the Dog Tag
Once you start the quest and receive your first objective, drive the Regalia west from your current location.
Stick to the main road and don’t drive at night.
Park the car near the blue area marked on your mini-map and look for a shack close to an electricity post.
Next to the said shack are several level 10 Garula you need to kill.
Objective: Retrieve the Dog Tag
A Garula from Final Fantasy XV can be killed with most weapons, but if you wish to take them out faster you should use fire spells, firearms and double handed-swords.
Keep away from its horns and attack from behind to maximize your damage.
For each Garula you kill, you will receive a Garula Tusk or Garula Sirloin. Make sure you keep them in your inventory because you can use them to craft more potent spells or to cook new dishes.
Now, that you have cleared the area, look for a Debased Coin x 1 on the ground, between the electricity post and the shack, then go around the small building to find the Dog Tag Dave needs.
The exact Dog Tag location for Final Resting Plains side-quest is pictured above.
Objective: Deliver the Dog Tag to Dave
After you find the Dog Tag, return to your car, and drive back to Dave at Coernix-Station Alstor in Duscae.
Talk to him, and hand over the tag to complete the quest and receive the rewards below.
After this quest, Dave moves south, to Cauthess Rest Area outpost, where he will wait for you with a new quest, named A Rocky End.
Final Resting Plains Quest Info
Description: “Yet another hunter has fallen, and Dave needs help retrieving their dog tag. Noctis agrees to venture into the wilderness and find the tag for the sake of the bereaved family.”
Reward: Elixir x 10
Post Quest Suggestion
Complete A Professor’s Protégé side-quest.