Regarding aux tanks there are multiple ways of doing it. Mine is set up rather like a diesel boat with multiple tanks. There is a draw & a return line controlled by valves which in my case are located on the floor directly in front of the drivers seat. If I wanted I can draw from one & return to the other. Why I would want to I don't really know but I can.
The aux tank is about 36gal & is half tank & half tool/storage box. Handy. Carrying a total of around 70gal gives an easy flat land with trailer range of over 600 miles. Nice to have that capability especially in areas where fuel is not on every corner, or even where there are not many street corners like headed to Alaska where depending on routes you can easily be 200 miles of less than ideal driving economy roads between fuel chances.