We just finished a trip that included 4 nights in the Estes Park area. We stayed at East Portal Campground. It is a county park with water and electric. They only have a few sites that can accommodate larger campers. We had A35 (I think) which would have handled a much larger camper than our 20' but it is difficult to get into. They don't recommend anything over 20' on the interior of the campground. They do have several sites up front that are accessible by larger campers.
The campground is gorgeous with reasonable amount of space between campsites. Deer come right up to the campsites and even let you pet them. Rocky Mountain National Park trails are only a few hundred yards away. The campground is located at the end of Tunnel road so no annoying traffic noises.
This was my wife's favorite spot of the 4 or 5 campgrounds that we stayed at this year on vacation. We will definitely be going back.
http://www.larimercamping.com/camping/Estes_Park_Campground_At_East_Portal_Reservoir/r/campgroundFees.do?contractCode=LARC&parkId=300101