Best setup I’ve ever used for all-in-one is the FishPond Wasatch Tech Pack vest. It has a decent amount of storage in the back that can accommodate enough for a day trip or overnight survival in warm or moderately cool environments. The problem is that FishPond discontinued the vest and doesn’t make anything that truly replaces it. You can still find a few for sale online. I run a 3L water reservoir and tube, all my fly fishing tools, it has a spare rod tube holder on the outside, drink bottle pouch on the other side, and enough rear storage to carry a rain jacket, fleece pullover and gloves, GPS, high calorie snacks, trauma kit, space blanket, fire starter kit, emergency signal kit, flashlight, headlamp, space blanket, and a few other small items. The front internal pocket perfectly holds my Glock 43.
Fortunately, FishPond came out with a good system with their Thunderhead submersible backpack and chest pack. They work well as stand-alone systems, but the chest pack also separates from the harness and attaches to clips on the front of the backpack straps. You have a fully integrated system. In terms of modular systems, it’s the best I’ve played with. I recently got my hands on both at the same time, and it convinced me to pick up the set for my next trip down to Belize or Mexico.