Most Wal Marts in the states allow overnight parking for RVs. I've never done it, as I'm still a wannabe nomad, but I understand it's best to arrive in the afternoon and check with the store whether they allow it. The ones most likely to say yes are the stand alones, not those connected to other stores in a strip mall. As long as you don't unpack chairs or barbecues and set up a campsite in the parking lot, they're usually ok. They do like you to be gone early the next morning. Also, depending on your route, you may find National Forests or Bureau of Land Management sites that allow dispersed camping up to 14 days. I know there are plenty of both in Nevada and Arizona. I don't know about the more northern states, but it's easy enough to google and find out. Happy trails, and we'll see you at the RTR!