Auxiliary Fuel Tank/Tool Box Thoughts/Advise

RDS 60 gallon aux gravity-fed tank/toolbox combo installed in the bed of my 2017 Dodge Ram 3500 DRW. A drip-drip-drip leak became a torrential rush after leaving the manual valve open after a fill-up. Stopping at a restaurant and there was a puddle of diesel fuel running down the parking lot after dinner. Turning the valve off stopped the flood of expensive fuel, but discovered the capless filler tubes of the 2016 and 2017 Dodge Ram's and the inner flapper valve is unable to stop the overflow of fuel through the open filler neck. Multiple check valve companies did not know if their valves would resolve the overflow issue either. Frustrated, I went old-school and looked for a solution that would cap off the stock filler neck. In 2016, Dodge introduced dual filler tubes behind the filler door; one to fill the diesel exhaust fluid (DEF) which had a filler cap and an open filler cap for diesel fuel and a flapper overflow valve in the filler neck. With the gravity-fed RDS auxiliary fuel tank/toolbox combo that RDS claims fits 2006 and newer Dodge Rams, those from 2016 and 2017 with open filler necks will have overflow problems that the flipper valve will not stop. I replaced the open-neck filler tube with a modified Ford filler neck and a standard filler cap. Old-school. No leaks. No problems. I was also concerned about the safety in a roll-over accident. If the regular overflow valve would not stop the fuel in an upright position, what would happen in a roll-over?