That Ford dash mpg readout/computer is just showing average mpg after each reset. Bearing that in mind, it doesn't take in effect whether you are hooked up or not. Knowing that I just use it as a general guideline since mileage can vary for many different reasons including fuel quality (cetane number or CN) or percentage of bio plus atmospheric conditions. The higher cetane number, the better permanence but as I've found, B10 to 20 wont give better fuel mileage. Then, add in an exhaust filter cleaning process which will instantly knock off 2-3 mpg on that dash gadget.
As far as fuel mileage - if you are towing $50K to mid six figure 5er, you need to suck up the fuel price sticker shock and enjoy (easy to say now that we are RVless this summer
) but 100++ buck fill ups are still happening since I still own the TV