Jeff -
I have been accused of being truck-obsessed (see the helpful post at
http://www.trailmanorowners.com/foru...uck#post46337), so skepticism is probably in order. But altitude, especially with a heavy load, does have a cost, and I don't think that a 3500 pound vehicle is going to be satisfactory in the situation you describe. A non-turbo gasoline engine loses about 3% of its sea-level horsepower for every 1000 feet of altitude, so at 10,000 feet (presumably an altitude within your travel plans), your sheer pulling power will be reduced by 30%. Your minivan's engine will be struggling, because it can't breathe. The engine doesn't care, it just cranks out 30% less torque and power, and keeps on chugging. But it means down-shifting, slow travel, higher engine and transmission temps, and added stress on all of the drive-train components, which can shorten the life of the vehicle. Beyond that, high altitudes imply steep grades, so your tow vehicle is doubly stressed. And the weight of four people and their stuff in the mini-van is another source of stress. This may be manageable in the flatlands, and some people do use 3500-pound vehicles there. But in the Rockies, I am skeptical. That's why I always ask the question you have already answered - "WHERE will you tow?" I leave it to you to check the Chrysler web site for the real meaning of that 3500-pound rating.
BTW, I owned a 2000 Chrysler minivan, and I loved it. And true to legend, at 100,000 miles, it spread a carpet of small but expensive transmission parts all over the road. I repaired it and loved it for another 50,000 miles - but it was an expensive experience. I never towed a trailer with it, but if I had been towing a trailer for some of those miles, I expect I would have experienced an earlier demise.
BTW again, I own a cabin outside of Estes Park, elevation nearly the same as yours. I have had actual experience towing the TM at these altitudes - and also at the Eisenhower Tunnel and the Vail Pass. Altitude and grade do count!
As for your final question, have you looked at full-size vans, as opposed to mini-vans? Plenty of space, reasonable MPG, and it is easy to get a 5000-pound tow rating.
Hope this helps
Bill