If you are going to Valley of Fire there is a free boondocking area as you climb the hill heading out of Overton on 169 called snowbird mesa or poverty flats. I checked on renting a sxs in town and they said the trails were outside the park sort of to the north west???
If you go to the boondock area I understand that you can ride the power lines to the riding areas. I didn't ride there but the rental place said that.
There are a couple rigs that are living there but it's fine. There is no time limit on staying because it's not blm. It's some sort of land left over from the days of the Lake Mead project so some people winter there.