Salary difference between full-time as opposed to prn status as an LPN

Hello, I am currently negotiating with my employer for an increase in my hourly salary since changing from full-time work status to prn status to accommodate my family schedule , which involves my elderly mother and my elderly mother-in-law. I have >27 years of continuous nursing experience as an LPN in the state of Va., which has included many duties that are normally required by RNs. I went into Correctional Nursing in Dec. 2008. My beginning pay rate was base weekend pay of $25.00/hr. I was making $25.45/hr. full-time working every Fr-Sa-Sun 12 hours shifts 7:30 p.m. - 8:00a.m at the time I changed to prn status.(that always ran 13--14 hours by the time shifts changed). When I changed to prn status, my hourly pay rate was decreased to $20.45 without prior notification. I have several co-workers that at the time where prn and had continued to make their full-time pay rate, which most were @ $25.00/hr. When I inquired to my Nursing Director as to why the decrease in my salary, she immediately forwarded the inquiry to the Administrator of the company.
Her response was that prn nurses did not make but the BASE WEEK DAY HOURLY RATE OF $20.00/hr. At the time I went with the reply, but since I have serious doubts that I am being treated properly. I have been working at the minimum since going prn 36-40 hours per week, but there are so many vacancies at the prison for nurses, I could actually work 7 days a week, every week. With this prn position there are NO BENEFITS included.
Please advise me the 'standard' pay difference for LPNs in VA for "prn" status as opposed to "full-time". If you cannot answer this please give me any suggestions as to dealing with this issue.
This is going back a while but when I was a LPN my benefitted job paid $23 and my PRN job paid $28. In my experience every PRN job I have held paid considerably more than the one with health benefits, vacation time etc.
Thanks for the reply. I totally agree.