Looking at the 2015 F350 tow ratings for the 6.2L gas engine, they show around 11,800-12,200 lb max trailer for fifth wheel towing for the 3.73 axle and 14,800-15,200 lb max trailer for fifth wheel towing for the 4.30 axle. I guess the first order of business is to find out what axle you have.
14k dry sounds too heavy for an older Big Country. Our previous 3950FB, which is one of the heavier current models, was running around 13.5k empty. Look on the driver's side for the GVWR of the trailer. That would be the maximum loaded weight allowed. For example, on our current unit, that number is 15,500 lb.
On newer trailers, not sure if they did it back in 2008, the dry weight of the unit as it left the factory is shown on a yellow sticker on the door frame. You can see it by opening the outside door(s).