Each state in the EZPass system seems to have their own rules on the transponder. I had NY for a year but then I discovered I could switch to Mass and have no annual fee.
There may be other states that have no fee or low fee but the tolls you pay are all the same.
I live in Ohio. I have Class-C RV but do not tow. You do need to pay attention to vehicle class when first signing up.
If traveling to states that have their own system simply put the transponder in the special RF shield bag. Depending on costs and travel plans you may want to join more than one system.