Hello and welcome to JustAnswer.
I'm sorry to hear about your situation and hope I can help.
It depends on the situation as to whether or not this would violate mandatory overtime laws. While at-will employers typically have a great deal of discretion in requiring their workers to work overtime hours there are some exceptions to this rule.
If the employer is requiring you to work 15-18 hours everyday in order to avoid making new hires than this would likely constitute a violation of mandatory overtime laws.
Additionally, requiring you to work 15-18 hours everday as an R.N. would also likely constitute a violation of the Occupational Safety and Health Act, since you would not be given a sufficient amount of rest in between shifts, which could lead to personal and patient safety issues.
If your employer persists with this requirment, I would suggest that you file a complaint with the California Occupational Safety and Health Commission and the California Department of Industrial Relations.
Here is a link to file a complaint with the Department of Occupational Safety and Health:http://www.dir.ca.gov/dosh/complaint.htm
And here's one for the Department of Labor, Division of Labor Standards Enforcement: