Hi,
good luck with your build, happy to share some pain and advice along the way! I used a 60 liter rectangular water tank from Front Runner outfitters and then the hose and spigot kit from them as well. You are correct, it is gravity fed. I put the tool box in the bed to be able to have a lockable storage. I drilled a hole through the bottom and feed the hose down into the bed. Conveniently there was a hole in the bed, so I aligned the holes. I then mounted the spigot to the underside of the bed. I then used a heavy duty eye bolt with large fender washers and made hard mount points inside the toolbox by drilling through the bottom of the box. there is one on each end of the water tank. I then used a 1,000lb ratchet strap to secure the tank. This allows me to take it out to clean, etc. I just fill it in place as it is to heavy to lift in and out when full.
happy to share some pics, do you have Instagram? easier pic sharing platform than this app. @ncat_explores