I stayed at a Walmart in Bend Oregon a couple weeks ago for the first time. However we called literally every campground within a half hour and they were all full. Even the expo center which has over 100 sites and a huge open field. We asked if we can just park in the field since the campground was full and they said no. I was with another fifth wheel, we parked side by side, opened the slides, brought out camp chairs, and drank some beers. There was about 15 other campers doing the same. One next to us ran their flipping generator all night for some reason, which was irritating.
I personally would only do it as a last resort. Although we sleep overnight in rest areas often. We leave at 8 or 9 at night. I drive until I'm tired, sleep for 5 hours then get up and arrive at our destination early in the morning. Not everyone's cup of tea but works for us.