I've seen very few people hit the axle ratings if they stay with the
truck's vehicle weight ratings.

Ford's Guide computes the maximum towing weights based on a minimal
truck, e.g., in the above case an XL trim truck without anything
added.  Moving up to a Lariat from an XL, and including a bed liner,
tool box, etc., could add 1,000 pounds to the truck's weight.  Two
real limits on the maximum trailer weight exist, computed as follows.

The first limit is based on the trucks maximum gross combined vehicle
weight limit (GCWR), which for an automatic diesel SRW F-350 is at
least 23,000 pounds.  If the truck weighs 8,000 pounds the maximum
trailer weight is 23,000 - 8,000 or 15,000 pounds.  This is close to
Ford's number, and is how they get their maximums.

The other limit is the gross vehicle weight rating (GVWR), which
limits the pin weight. This is the limit most fifth wheel and
gooseneck rigs hit.  An F-350 diesel SRW 4x4 LB has a GVWR of 11,500
pounds.  If the empty wet truck weighs 8,000 pounds, the maximum
weight of your payload, which includes pin weight, passenger weight,
and the weight of everything else you carry, is 11,500 - 8,000 or
3,500 pounds.  

If your pin weight is 1,700 pounds, everything else must weigh less
than 3,500 - 1,700 or 2,200 pounds.  Assume you carry four passengers
at 175 pounds each, a hitch weighing 200 pounds, and another 300
pounds of cooler, tools, dog, etc., for a total of 1,200 pounds, and
you are still 1,000 pounds under the truck's GVWR.

I will note that a 12,000 pound trailer's pin weight should be between
15% and 25% of your trailer's weight, or between 1,800 and 3,000
pounds.  Your 1,700 pound pin weight seems low.  Using 20% of the
trailer weight on the pin, which gives a pin weight of 2,400 pounds,
you're only 300 pounds under the truck's GVWR.

If the truck actually weights 8,300 pounds wet and empty, rather than
8,000 pounds, and we use 20% pin weight, you've hit the GVWR.  Or if
the weight of the trailer rises much, e.g., after you've stocked it
and filled the tanks, the pin weight rises and you're again
approaching the GVWR.

When I read people's comments about SRW vs. DRW, most who have towed
with both say the DRW is much better, not because of the weight
ratings but because of better stability and handling.  For the above
truck the DRW's GVWR is 13,000 pounds, an increase of 1,500 pounds but
the truck weighs an additional 200 pounds.  The DRW would provide a
net 1,300 pounds of additional payload, a more comfortable margin than
the SRW's 300 pound margin.

I think you need at least an F-350 DRW truck to get the margin you
need.  Could you get by with an F-350 SRW?  Probably.

You'll find lots of good information, including this sort of
discussion, over on www.TheDieselStop.Com.

Bob