Four Corners area is beautiful and very nice camping/boondocking country. We lived in the area for several years and camped a lot in RVs and backpacking. Too many good places to list but here are a few suggestions. Stop in at a NFS/BLM District office and pick up NFS maps for all the areas you will be in and they have campgrounds listed.
1) Transfer Campground NE of Mancos, CO - a NFS campground above the West Mancos River; any place you want to pull off the FS road between Transfer and Windy Gap which is some 15 miles past Transfer - gravel road all the way but normaly in good condition. Check it out in your tow first.
2) Burro Bridge Campground - another NFS campground about 19 miles off the road between Delores and Rico, CO. Great fishing in the West Delores River which runs past the campground.
3) Numerous places to pull off and boondock or hit NFS campgrounds between Rico and Telluride, CO.
4) Two NFS campgrounds between Pagosa, CO and Wolf Creek Pass.
As you must know, many places along the highways there with access to NFS properties and camping areas. Also check out this site for more: Free camping in Colorado | RV camping, Car Camping, Tent Camping
Beautiful country, enjoy and be sure to take your fishing gear - rivers and streams everywhere.