Considering your 350 is pulling often that is a good number.My ‘13 F350 drw crew cab 4x4 has averaged 13.2 in the last 55,000 miles. Spends a lot of its life in the 25-32,000gcvw range though.
‘07 dodge sprinter long wheel base high roof, last 5,000 miles has averaged 18.4 mpg. Over half that is highway speed which makes the mileage tank for this one.
‘15 GMC 1500 crew 4x4 5.3L over last 21,000 miles averaged 14.5. Almost all town running.
‘00 Ecursion 7.3 4x4 over its life with me of 159,000 miles averaged 16.2 even though it had a lot of engine work and a little bigger tires.
‘16 GMC 2500 6.0L crew 4x4 over 6,000 miles averaged 10.1 with most town running. FIL didn’t keep this one long because of the shit mileage and he didn’t need the 3/4 rough ride for a putt around town truck
Our best average over several years hunting out west is 12.4.
Not near that heavy though.
R