For those who own the game but never played it and just started now: From the older guides I refer to this video which just now helped me on the PC.
Take note that on 6:08 in the video a guy is being missed. Looking from inside the white truck, go to the right through the radiated water and then go another right. There will be a guy in the water with a clip of gold ammo.
Advice: At the end, do not go to Bourbon but go past him and towards the body shown at the end of the video. The achievement popped for me at that stage. This ensures that you have enough time to collect it all.