We have a 34' Newmar Ventana LE with the 340 HP 5.9 liter Cummins engine, and the 6 speed 2500 series Allison transmission. I love the combination. The transmission is the smoothest shifting of the 5 Allison's I have had, and I can climb hills as well as our 41' Revolution with the 400 hp 9 liter CAT engine. In my opinion, one reason it does well on hills is that it is governed at 2800 rpm instead of 2400.
Regading the 260 hp, we had a 36' Monaco back in the 90's with the 230 5.9 liter Cummins, and lived in it for over 2 years. We were very happy with the performance of that coach. The weight on that was about what you quote for the Expedition you are looking at.
Ed Headington