Like RichM, we've found Wal-Marts (in most neighborhoods) to be good and 24-hour grocery stores to be a good second choice. Both can be easily located, day or night, if you have internet access in the truck that is not tied to the truck stop internet services.
Wal-Marts are especially good because most have roving security patrols in the lot day and night. That helps keep the riff-raff out. Plus they are handy places to buy the stuff we need to resupply the truck.
We avoid bad neighborhoods like the plague. Signs of a bad neighborhood come from the residents themselves. They put bars on their windows, barbed wire on their fences, litter in their streets and graffiti on sign posts, bus benches, building walls. They support business like check cashing stores, pawn shops, and bail bonds houses.
I learned a little about gang graffiti, signs and colors when I worked in a prison (one of my technology gigs). Asian, black, white, hispanic; it all reads the same to me now. If you see it, drive away.
These neighborhoods have Wal-marts and 24-hour grocery stores too. Before setting out to one of those, and if we don't know the city, we'll use our mapping software and the internet to find upscale stores that are generally found in graffiti-free neighborhoods.