It's perfectly legal and perfectly safe depending on location of course. I have stayed with my Ursa and a few pop top vans in rest areas with no problems. The biggest problem will be noise in any RTT since they don't block it very well. People will walk by at 3 am admiring/discussing your rig with no respect for your sleeping above. Kind of annoying.
Personally I'd use the apps available to find a place off the grid. Even the back of a Walmart/Lowe's or any closed store is preferable to me.
I use FreeCampsites.net & iOverlander for unscheduled landings. Works well for me.