My wife and I used to own a little grocery in a small Colorado town and very often campers and hunters would ask to fill up with water. Absolutely was the standard answer. Its good for the customer and good for business. We have also stopped and asked homeowners if we could use there outside spigot to fill up and never been turned down.
Park of the allure of camping is meeting new people anyway and locals will often point you in the direction of activities and events you didn't know about. Planning ahead is best but in a pinch, stop and ask. You'll be glad you did.