Is it okay to drop my tip out beds at rest stops or Walmart?

Not recommended. You can ask at the Walmart you're staying at if it's ok, but the general feeling seems to be you can't look like you're camping - so no slideouts, awnings or tipouts deployed, no BBQs, etc. At many of these places - rest areas especially, but also store parking lots - I just don't think it's safe to sleep in an exterior bed, even if attached.