As I have said before, we have the RDS 50 gallon (actually holds 48), bought from Northern Tools with installation kit, and costs about $600 complete. I installed one in a friend's F-350 in about 45 minutes. This tank of ours is on its third truck and never a problem. Yes, it is gravity feed. I use the petcock on the tank, but we put a on-off petcock on his in the wheel well. You might start here and go from there.
https://www.northerntool.com/shop/to..._541559_541559
PM me and we can talk if you need more info.