The MVUMs only show where dispersed camping is allowed, they do not show where there are places suitable for boondocking. Only discussion with the district office and scouting reveals where you actually might be able to camp. Don't forget the BLM lands. There are millions of acres of these lands to camp on, most of which you could be really alone. But again, it takes a little work looking at BLM property maps, talking to BLM Area personnel, and scouting to find the gems. Road condition can be a limiting factor.
I was in the Big Horn Mtns recently and wasn't surprised to find that the Forest Service had to number and regulate the dispersed sites along Forest Service roads that had been used for decades. These newly numbered sites are the only dispersed sites where campfires are allowed and many of the sites that have been overused are now closed. Just an indication of what can happen when places are loved to death.