Thermoguy,
If you want to tow upwards of 10,000 lbs., you can forget about the new gasser.
I'd recommend an older diesel unit before the DEF was required, which I think means 2008 and before, but not sure on that.
Even then, make sure the rig can tow what you want. There are DPs that have relatively modest tow ratings, and others with much greater tow capacity.
Do you have any preference for floorplan? It's often said that is one of the most important things once you've decided on the mechanical aspects.
Good Luck in your search. I'm sure you're ideal rig is out there somewhere.
~Rick
