Forced To Sleep In Facility Unpaid In Us Pa

I have been working for the same company for approximately 9 years and I have brought the issue up with our union(no answers) as well as a couple other issues, and management began doing random pop ups every shift following for 2 months to find reasons to terminate me. I work a 40 hour shift every week over a period of 3 days however I am required to stay on site every 2 nights (16hrs). I am not permitted to leave the facility, drink, play video games, have people hang out,etc during this sleep period without being terminated. I am on call as well during this period if a incident happens requiring hospitalization I am required to wake up, punch in and go to the hospital or assist the other staff in the event of a crisis. They also do fire drills occasionally while we are sleeping requiring us to wake up and do our job then go back to sleep. I am in the mental health field and I take care of people with disabilities mostly violent individuals. There has been numerous occasions where I was unable to sleep due to a individuals having behaviors(screaming, yelling, and swearing) that didn't require me to intervene but prevented proper sleep, I have also been woken up before to police having to intervene however I didn't have to wake up as the individual wasn't hospitalized. I get paid a additional $0.50 a hour while I am awake working(daylight hours) however this amounts to $20 a week before taxes for 16 hours of being on site. Doing the math for just 5 years and if I got minimum wage not counting towards overtime I would be owed almost 30k before taxes. I feel like this should be illegal but I am always told the union handles our contract and this always gets shot down. I would give up the $0.50 extra if I got at least minimum wage to sleep on site. Everyone I have talked to outside the company says it isn't legal but it's been this way for 9 years and I feel like someone would have sued them and won by now if that was the case. I am in the medical field, I am not salary, I get a hourly rate and overtime for hours over 40 in a week, I am on call on site and I am not permitted to leave. I am given 8 hours a night to shower,sleep,get dressed, I am provided a staff room with a bed to sleep on during these hours, I receive a $0.50 shift differential during the hours I am working but not while sleeping. Is this legal in the US?