There are a number of free over night stays available. An app allstays is one source. I generally don't like the truck stops because of the tight conditions and noise. I have stayed in a number of rest areas with no problem. Seems those in the souther tier of states SE especially, are more receptive. North east and west coast can be hostile to that. Walmart is a good place BUT check ahead and as stated, park where they ask. Don't camp out with grills, lawn chairs, etc. When you leave clean up as though no one was there. I will put slides out since some might be needed to access parts of the vehicle. If at a Walmart, go shopping. A nice rotisserie chicken saves cooking anyway.
Don't do as I saw in Meridian MS last summer. 3 Rv's, 2 motorhomes and a 5th wheel, circled the wagons, set out all their gear, took up a huge portion of the parking lot and made a bad statement about RVers. Be courteous and make a small statement. The camping opportunity you save might be yours the next time through.