I probably should have been more specific: what I want to charge is a deep-cycle battery for a couple of hours using a fast (26 amp) charger. I don't want to leave the battery and charger on the ground next to an outlet unattended, so that means parking my car next to an outlet so I can lock the battery and charger inside with just the power cord plugged in outside.
Probably wouldn't hurt to be even more specific if you want a solution. The answer to your original question is
NO, don't go around expecting to get power from places that don't offer it. Who cares if they will or won't allow you to use on outlet, you don't know for sure it's allowed, assume it's not.
Your question is kinda the opposite of the general attitude around here of being mostly self sufficient. From that angle, your best option would be to have a way to charge your battery using your own equipment and if utility power is available to you then you can take advantage of it to top everything off. So I'm not saying don't plug in if you can find a way, I'm saying don't be in a place where you are screwed if you can't find a plug in.
It's obviously important to you that you have battery power available, but not important enough to look for a way to charge it without hoping for a haphazard chance that you find someone willing to let you use an outlet?
One idea is your vehicle, assuming you drive at least some of the time you are camping. Perhaps you can set up a system to charge off the vehicle while you drive around. Despite what some might say this does not necessarily have to be expensive.
Someone mentioned a generator, I understand there are some quite small ones available that might do just fine for charging a battery once a day.
Someone mentioned solar already. A little less predictable in my opinion in the event you get a lot of cloud cover for more than one day in a row, but something to think on.
There are some really smart folks on here, you tell them what size battery and what you have to work with, I bet they will suggest some way you can charge it yourself without having to beg for an outlet somewhere.