Ron,
I found this in the FAQ's section on the PA EZ pass website.
"Can I use my transponder in my RV?
If you have an RV, with a gross vehicle weight over 15,000 pounds, you will need to open a commercial account. If you have an RV that has a gross vehicle weight of 15,000 pounds or less, you may add it to your personal E-ZPass account. Please be prepared with your weight, tires and axles when calling to add an RV to your account. You may require a separate transponder programmed for the specific weight class of the RV."
You should be able to find your gross vehicle weight on a data plate in your RV.