Walmarts are *usually* amenable to letting you stay overnight. There is a yahoo group for Walmart RVing that maintains a list of which walmarts you can/cannot stay at. No matter what- it's courteous to go in and ask (even when they allow it). I don't have a generator, so I'd be looking at a very small window for when I would be comfortable doing it. There are general guidelines that "the walmart overnighting community" suggest you follow - protect the asphalt as it can be easily damaged by stabilizers, no slides if they'll intrude on other parking spots and, as mentioned, always ask first. These seem to be hotly in debate whenever they're mentioned. Noise-wise, i would guess that it is hit or miss.
Some rest areas allow parking and rest for a few hours. I'd just try to be courteous to truck drivers as their stops are by law and that's their livelihood. But, I would also think that rest areas would be quite noisy.
Various other large retail establishments often times allow overnight parking- Home Depot, SAMs club, and even Cracker Barrels. As mentioned, lots of truck stops will allow you to park for the night, but that seems like it would be the loudest option of all.
Some folks will even stealth park on public streets if they're in van-like camping units. This gets dicier and isn't something I would entertain (not to mention - there is NOTHING stealthy about my setup
If you're traveling highways, often times there are campgrounds close to them that are relatively inexpensive ($35/night) and are convenient for folks traveling thru. Two that I've been to were level pull thru sites that we were able to stay hitched up.
Personally, I have no problem with the Walmart/legal rest area thing. We don't do it mostly due to weather and wanting A/C (and we don't have a handle on dry camping).