I just got a new 2009 E250 with a 5.4. It got over 15 mpg from LA to to Kennewick. One slower stretch I got 16 mpg. However it was empty with a standard top. It will be interesting to see what happens to the mileage after adding 2500 lb of payload and a 24" high bubble top....
I have been doing some reading and there may have been some fuel economy changes over the years with the 5.4, fine tuning of transmission, air conditioning, alternator, cruise control, etc.
As far as handling steep hills, it will do close to 55 in 2nd gear. Haven't checked the gas mileage under those conditions.
Oh and Silver350 is probably quoting Imperial gallons or something weird