I think that GM has had a real hit with the Duramax/Allison combo since day one. It is quieter than than the others and they seem to make it better and better each year. For 2011 the factory exhaust brake seems to have gone over big - search for Rumble in the Rockies where it smoked the new Ford - both up and down hill.
Good luck with finding both your truck and trailer -and yes you are right that the 25P came out in 2007. As for good deals on new rigs you might try Dave Smith in Kellog for the truck and Thunder RV, or Sumner RV for the TT all three have been known for selling below retail.
07 Chevy 2500HD 6.0L ,6 speed auto, 4x4, Blue Granite Metalic
05 Arctic Fox 22H