For a rig that size you need a truck with a 10000 Lb and up rear axle rating.
The F450 pickup comes with a 9999 RAWR and good for around 6000 lb on the rear axle.
The '04 and older F450 pickups were much stronger rear suspension with the 7.3 diesel had a 11000 lb RAWR..
As some suggest a 450/4500 and 550/5500 cab & chassis has 11k to 14k lb rear axle and will have no problems with the load you will be carrying with that unit.
Some of the lite weight RV hitches like a Reese/Curt/B&W/etc won't be up to handling a 28000 lb draw or a 6k-7k lb pin weight trailer. The lighter weight SAF Holland or Fontaine or the Binkley on a flat bed would do the job.
AS far as the size of your work truck goes much depends on what your work is. (flat bed or pickup bed) ?
