I've slept under "No Overnight Camping" signs plenty of times in our cargo trailer. I've even been given tacit permission by shop owners when the signs were posted due to local laws. It's a bit of a blur. We try to stay outside the city since there is so much public land out there to stay on anyway, but when in a city we will check AllStays.com for reports on the local Walmarts (most still allow sleeping, its just at the local manager's discretion), look for nearby Cabela's or similar. We've slept at construction sites too. You basically just have to figure on where you are likely to actually be rousted.
I've never heard of an arrest/tow unless the vehicle couldn't move. I've been doing this for nearly two years now, and nobody I know has seen that either. You usually will get rousted instead. If your vehicle is disabled and you are parked illegally, you may have a problem. If your vehicle might end up disabled, most auto shops will let you sleep in their lot while they fix it. This works especially well if you tow a trailer (we use a cargo trailer as stated above).