My '15 F350 here in CNY nets:
City Ave winter: 10-14
Hiway Ave winter: 13-17
City Ave summer: 13-16
Hiway Ave Summer: 16-19
Towing so far (snowmobile trailer, 4 place V-nose enclosed): 10
No long trip yet w/ RV so cannot give you a accurate mpg figure. I run at 60 in a 55 and 70 in a 65.
As for the diff between a 250 and 350, not going to get into a debate about it, BUT what could happen in the event of a accident and one rear ends someone else w/a truck that's overloaded is that some sue happy att'y is going to go thru things w/a fine tooth comb and while ins will probly pay out, end up dropping the owner. So yes a 250 can be overloaded rather quickly, but a 350 and up tonnage truck can be equally as overloaded. Heck, how many 1/2 tons do we see overloaded? Either way one goes, the best offense is a great defense and allow plenty of gap between you and the next guy. Sure some yahoos will pull into the space, so you'll have to adjust again and again.