Using the restroom shouldn't take more than a minute or two. Surely a quick glance out the window would be enough to determine that no customers were imminent when one needed to go. I work 12.5 hour night shifts, and if a nurse has time in her schedule to eat then surely a convenience store clerk can find a slow time to have a quick bite. I can understand that not knowing about your schedule in advance can be frustrating, but your manager has limited control over the actions of other employees. Have you considered that one of your coworkers may have called in sick leaving him short staffed, leading him to extend your shift to guarantee coverage?
I've been called an hour before work to be put on standby, with the knowledge that work could call at anytime that night and I would be expected at work within an hour, but that if I didn't get called in then my paycheck would be short 12 hours of pay. I have also been called with requests to come in 4 hours early to work (16 hour shifts are exhausting), or to come in on a scheduled day off.