Look here-
http://www.ford.com/assets/pdf/towin...RDUTYsep09.pdf
looks like a 350 maxes out at about 17,000 lbs depending on options. But what will get you is payload.
payload is the weight in the bed- max about- 3000-3800. many of these raptors are over the. but pay load includes passengers.
Remember- the trailer manafactures are all ways light, and you will pack much more than you think.
For me filling my water tanks adds 800 lbs alone. other camping stuff 500+ easy.
If its a good dealer maybe they will let you go to a local scale to see what it really weights. If so measure the weight of each axle of you truck.