Several people have stated to weigh the truck. I agree with that, but the 5er needs weighed as well loaded as it would be for a trip. Our 14,000 gross Jayco weighs 11,600 on the road with a 2600lb pin. Quite a difference from the 14,000 rating.
Tire ratings are very important here also. Someone mentioned the 245/75r16 size. I went to a 265/75R16 Transforce because they are rated at 3415 lbs. Watch out for AT tread styles, they are often rated much lower that the same size in a highway tread. After 13 years your truck will not have the original tires, make sure you are running a LT E rated set, not a P metric set somebody sold you over the years.
I would tow the new fiver and see how it does. If a different truck is then decided on the 7.3 will bring very good money. The problem with finding used duallys is that unless they were used for private use with RV's they are beat all to hell with higher mileages.