Colorado is a big state and you didn't say which part you're looking at, but one of my favorites is the Piedra river between Durango and Pagosa Springs.
There is a National Forest Campground on the west side of the river about a mile north of the highway, but on the east side of the river the road follows the river (though often high above it) for about 15 miles. There are several boondocking locations along the way as well as the end of the road at a bridge across the Piedra.
I would suggest however that you stay in the NFS campground. Access is easier than towing a long trailer on the road on the other side and it does provide a vault toilet as well as potable water (central hand pump).
