We always ask permission (generally at the customer service desk) and ask where they'd like us to park. Almost always it's long the edge of the lot and as far away from the store doors as possible. We put the slide side away from the lot so it won't be in any traffic, and use the generator only if it's hot enough to need the AC before bed. If not on the edge of the lot, we try to get next to a light pole and put the slide on that side.
Walmarts are not the only option - most big box stores will tolerate your rig overnight. Last month we asked a store manager if we could spend the night in his lot. He said he couldn't authorize that as they didn't have security or insurance. He then also told us just where in the lot would be the best and quietest place to set up and assured us that we wouldn't be bothered. It was one of the best unauthorized night's sleep we've had.
Most Cracker Barrels have "rv" parking spaces that you can spend the night in. If you get there after the dinner rush, there's usually space. And breakfast is just a few steps away in the morning!
Just don't act like you're entitled to spend the night, and most folks will be happy to host you. Just remember - you're not camping, you're parking. Take up as little space as possible, and don't "set up camp".
al
