According to the 2005 Ford towing guide, it depends on your axle ratio
Auto trans, Std & 5th wheel towing: 3.73= 9,000, 4.10=11,000,
Manual Trans, Std & 5th wheel towing: 3.72= 8,000 4.10= 10,000
Lots of fine print to read thru, but those are the basic numbers
http://www.fleet.ford.com/resources/ford/general/pdf/towingguides/2005_Towing_Guide.pdf
Yes, toy haulers are heavy for their length, and in my experience they pull heavy due to their typical kick-up at the back.
That 5.4 is going to be really working hard.