There is an easy boondocking are just outside of Grand Lake and Rocky Mtn National Park. Very easy access, and multiple sites that can handle larger rigs, although the first one is the most ideal (great views, and huge area). I stayed in that first space just a few weeks ago:
http://www.van-tramp.com/wp/boondocking-near-grand-lake-co/On the South-East side of the National Park there is the Beaver Reservoir area. Shouldn't be too hard to get yourself in, but turning around a truck with 30+ feet of trailer may be another story:
http://www.van-tramp.com/wp/beaver-reservoir/If you are way down there near Grand Junction, then you can go to Rabbit Valley BLM (Exit #2 of I70, head south). I've only overnighted at the signs, but it does appear to have a good sized dry camping area in there.
I have a few others up in the RMNP area, but I wouldn't recommend a 35' trailer try to get in there.
