River Mountain Lodge- the owners (as with any condo complex) can choose any property management company or do it themselves and advertise thru websites such as vrbo.com/homeaway....
With that said, there are a lot of owners that rent with the property management that is onsite...you can try them at 970-453-4711.
Trails End vs Lift- without seeing pics. More likely to have a larger unit (square footage wise) at The Lift.
Trails End is a great complex for ski in/out but since you are coming in the summer, you might like the larger unit at The Lift. Surface parking, most likely stairs up to your unit. The Lift has access to the Columbine pool which has a heated pool and indoor/outdoor hot tubs. Short ez walk to main st. (go thru Main St. Station).
Other complexes worth a look near The Lift are
MountainWood, Timbernest, SnoDallion, Columbine Condo, The Corral.