Nothing like enough truck for the job. The 8.1 is a strong engine, but like any gasser it takes a lot of gasoline to pull a trailer.
I am a firm believer of a diesel engine for heavy work. Out 6.7L F350 Dually will typically average 9 to 10 MPG, running 63 MPH with our 40', 18500# 5th wheel. The truck running solo (weighs about 9200# loaded) will do 15 to 16 mpg.
Last fall we traveled across Tennessee headed east and maintained 9 mpg.
The Ford tank is 37.5 gallons and we added 40 gallon auxiliary tank. It makes traveling so much nicer when you are not having to watch for fuel every few hours.
Ken