Disciplined for FMLA excuse

I am currently on approved intermittent FMLA leave for an immediate family member. My immediate supervisor is very much against any leave of any sort. Since I am a salaried employee who is not eligible for overtime, I have offered to work evenings and weekends to keep the work up AND also be charged my accrued time for the time I take off. While my boss complains constantly about my time off, he refuses to allow me to work any evenings or weekends or holidays. All other employees are allowed to work holidays at their discretion, including all supervisors (which are men) except me. When I asked my boss why he told me I would be generating overtime. I asked both him and HR if my position was eligible for overtime and they said no. HR responded that if I work less than 40 hours a week I must use accrued time. If I work over 40 hours a week, too bad. Approximately 9 months ago an attempt was made to discipline me for not providing an excuse from my family member's doctor for using one hour of FMLA time. At that time my boss told me not to worry about it, but denied telling me that when HR got involved. I got the excuse and received a letter from the employer's attorney stating that from now on every time I use FMLA time I must provide an excuse within 14 days. Not a problem. Now HR and my boss have decided that I must provide an excuse within 3 days and say they have no knowledge of the attorney's letter, of which I have a copy. My family member's doctor was out of town the week I used 8 hours of FMLA time and I was having trouble getting the excuse. Prior to the 3 days being up, I emailed both my boss and HR informing them of the issue and I would be about 12 hours late getting them the excuse. I got a discipline letter for being late with the excuse. I asked HR about FMLA regs and he replied that the employer can make any rule they want and I have to follow it (they informed me they just made this rule effective 3 weeks prior when I used FMLA time). I explained I did everything I could to obtain the excuse within the time period but was late due to the physician being gone.
This is just one more incident in 18 months of harassment because I am using FMLA leave. My boss has convinced upper management, including HR, that my family member is not really ill and that I am lying about everything. HR has received all the required paperwork and everything is in order. HR and my boss went so far as to have a meeting with me for HR to tell me that morale was bad at my workplace because I was on FMLA leave and I needed to do something about it. He said that other employees felt I was getting special treatment. I explained I was using my accrued time just like they were, but for a specific reason. They told me I had to improve my attendance because morale was bad. I have worked 22 years with impecable record and evals and the minute I used FMLA the harassment from colleagues and subordinated started, condoned by my boss. Am I correct is assuming there is nothing I can do to make it stop? I have documented everything and realize they are either trying to get me to quit or fire me for trumped up reasons. Do I need to wait for termination to really do something? I can't believe this is happening.
Thank you very much.