Liftime average on the 6.2 is 14 over 20,000miles.
Lifetime average for the 5.4 is 15 over 26000miled

Epty the 5.4 is 1000lbs lighter plus the manual trans probably is most of the difference.

With the trailer the 6.2 gets alot better mpg and does a bit better on the hills. I do much prefer towing with a manual; and usually tow with the 5.4 unless i need the crew cab