In a rest area I try to find a parking spot where we are away from the truck parking spots to reduce the noise and diesel fumes. It's not always possible to find this kind of spot.
My first choice is to find an inexpensive campground near the highway to stop for the night. Next would be a large truck stop with a separate area for RVs. Rest Stop is the last resort.
We have stayed (with permission) in a large grocery store parking lot, church parking lot, gas station (leave before they open), down a side road, large pull off area along with a few semi's...etc.