Has anyone with an aux tank tried filling the main tank when the main tank is boiling? That should cool it down and maybe stop the boiling??? Of course that isn't a solution but just a way to test the theory.
@TheGrrrrr, maybe you could just run the return fuel line through your shower heater on your roof rack and get both cool fuel and hot water...
I like the logic on the aux tank, but I think someone tried that and found that the transfer pump will immediately disengage because of the pressure levels in the main tank. If that could be overcome safely, it might be a mitigation strategy.
As for my water tank on the roof, it might already be hotter than the fuel. I didn't buy it to heat the water, just carry it, but heat the water it does!