I have a 3200 and the rear tank is for the generator. I tend to keep that tank kinda low when I don't plan on using the generator much and put most of the fuel in the auxiliary tank. When I need more for the generator I just pump it in. If I don't use all the gas I can pump it out of the tank and put it in the wife's car. This way the gas never gets too old.
2016 Dutchmen Voltage 3200
2017 Ford F-350 Superduty, 6.7L Powerstroke, 4x4, Crew Cab, Long Bed