I know you want a gravity fed system but I'd encourage you to consider a transfer pump. I've been using a 13gpm pump for years on the 91-gallon RDS in the bed of my F250. I run all the fuel through sediment and water-removal filters, so I know I'm putting clean fuel in the OEM tanks. I also put a long-enough hose on the pump to reach other diesel equipment, such as tractors etc. It makes the system very versatile.