Thank you all for your responses. I expect our concession trailer to weigh about 4,000 pounds. The 3,500 pound trailer weight limitation on some of the sprinters can be upgraded to 5,000 pounds, so that covers that issue.
We are definitely not interested in any naturally aspirated gas RV's. They are quite bad in the mountains.
From my research, I agree that the I5 is the way to go. More reliable, easier to maintain, and better fuel mileage. I would add a tranny cooler for sure. And the $10K we save by not buying something newer will go a long way towards any eventualities.
Cheers