I'm sure it has to do with the in tank pump but why can't you use the same system that Ford had with their dual tank trucks :h ? Both my 1990 and 1992 F-150's had dual tanks that all you did was flip a switch on the dash. The only down side was it had fillers on both sides on the truck.
I thought you could now longer use gravity feed tanks? The two main reasons being if a fuel is cut they will empty out, and they also can over fill the main tank. Now if the fill tube is installed into the main tank say three inches in that would keep it from over filling. But if the fill line is taped into the filler neck that will cause issues.
I know on any vehicle that uses a return flow system you should never fill your tank to the top or into the filler tube. I cost us over $800 in repairs on the DD VovloS-80. Seems the e-vac system became over saturated with raw fuel and had to be replaced.
Something else to think about is the return line coming from the injectors will always return to the main tank so my idea of using a switch just got a lot more complicated. You would need some way to return the fuel to the tank it cam from. that means more valves a more cost, I'm beginning to see why these tanks cost so much! I bet the oversize replacement tank/tanks Transfer Flow sells are going to be the best solution for you.
LinkGood luck OP
2015 Ram 1500 Laramie Crew Cab SWB 4X4 Ecodiesel GDE Tune.