Can I sue my employer if I found out he had a camera in the ladies room and was watching all the woman?

Yes, you almost certainly can sue: what you describe appears to be blatant sexual harassment (as well as an actionable invasion of privacy). You should speak with an employment law attorney--in cases like this, the injured employee can often recover attorney's fees as well as damages, making a lawsuit very cost effective. If you decline to sue, contact your state's equal opportunity or equal rights agency/commission.