We spent last spring and summer in Utah and Colorado. I've marked all of our boondocking spots on this map -
http://goo.gl/maps/ZhlhY Read the short descriptions to make sure that the spot be okay for your trailer.
Here's a good resource for National Forests -
http://www.fs.fed.us/locatormap/ Click on a link on the map to get the forest web page, then click "recreation" in the list on the left side. Click "camping and cabins" then "dispersed camping".
The spots that come up are not the only places where you're allowed to camp but it'll give you a good start.
Dump stations -
http://www.sanidumps.com/sanidumps_usamerica.phpHave a good time!