Flatbed,
The 2005 LLY is a good engine and the five speed Allison is a great transmission. My 2006 LBZ with 215k miles is by far the best truck that I have ever owned and I have had some very good trucks in the past. With only 20k miles on the 2005 dually it isn't even broken in yet and should not give you any problems for a very long time.
Additionally, at one time I had a 16.5 foot slide out camper that was old and heavy and my 2006 truck handled it just fine so the 05 should be OK also. You asked about Ford 6.0. All I will say is that if someone gave me one I would know that they must really hate me. They make good boat anchors.
*2006 Chevy, 3500, 4X4, DRW, (LBZ) D/A, CC, LT, 252K Miles, 19.5" Wheels, Mag Hytec Transmission Pan and Differential Cover, SS Grill Guard, Racor 2 micron aux fuel filter, 100 Gallon Aux Fuel Tank, using Edge Evolution, Predator Diablosport, Kennedy ECM tune, Fitch Fuel Catalyst.
*2006 Four Winds, Dutchman, 36', RV, D/A, 5500 Kodiak.
*1993 Harley Davidson, Turbo charged, Springer Softail.
*2007 Pontiac Solstice.
*2005 Jeep, Liberty, CRD (diesel).
*Full-timing in USA, see America first.
BUY AMERICAN or CANADIAN, NOTHING from CHINA .