I have an 04 F550 Lariat towing a FR Sterling 32RL. I get 12 mpg when towing. The truck just turned over 60 miles. I am the third owner, the original towing an 8 horse trailer between Toronto and Kentucky.
I change the oil every 5000 miles, and regular filter changes. I do not mash the gas pedal (diesel) but the engine gives plenty of power. Your differential is likely different so off the line acceleration will not be the same.
When got the truck I changed all the fluids engine and tranny with all the filters. I suggest that you do the same.
Good luck have fun.
2004 Ford F550 6l Lariat SuperCrew, Fontaine Bed,
Link Systems Air Ride
2011 Sterling 32RL with Michelin XPS tires