1) Legally they can't tow you while you're inside the vehicle as that is tantamount to kidnapping. You don't need to worry about this due to any vehicle occupancy laws while you're inside. Only worry about getting towed while you're away if you've parked illegally (as you would with any other vehicle).
2) You cannot rely on Wal-Mart parking lots. Everyone thinks that's true but it simply isn't. Many Wal-Marts don't allow overnight parking, and others are in cities with ordinances against overnight parking (meaning police look to enforce the law there first, because everyone thinks Wal-Mart is the place to overnight park). In fact even where it is permissible I avoid parking at Wal-Mart simply because everyone defaults there and it can draw attention.
I've been living in my Class B RV (bigass van) full time on the road for over a year now. I never plan ahead, I just go somewhere and use common sense, and I've never been disturbed at night, towed, ticketed, or threatened with arrest. I do have to talk to police occasionally. Just be honest and they shouldn't hassle you. Although my van is not at all stealthy, it does meet any city parking requirements, so depending on the size of your RV that would be your only restriction beyond mine. Find street parking out of the way somewhere. If you go to a box store parking lot, make it any one other than Wal-Mart. Lowe's is a great choice. They never care about overnighters and there's always a strong wifi signal.
