I have had an EZPass since the beginning when they came out. Actually, I have 9 EZPasses on the same account right now. If you go online and register the vehicles with the current license plates, you should have no issues.
I was told many years ago, keep the vehicle registered online on the account and when you pass through the tolls, they will manually "adjust" the class you are in when you cross the scales. In all the years, I have not had a problem.
I now use the same transponder in my '10 Revolution 43' with tag and pulling a large trailer at 14,000lbs and have had no issues. They have adjusted the class and I have been deducted the correct amount from my balance each time.
I also have a large toter home with a 44' trailer and use the same pass in that vehicle with no issues.
Unless you are a commercial over the road truck, they have told me that they do NOT want you using the blue EZPass transponders. Plus the balance you must carry on those blue transponders is absolutely crazy!!
And, you are right about the customer service people, they can be helpful at times, but they don't always have the correct information.
And yes, I have used the same pass in my motorhome in PA, MD, OH, IN, IL, NY, MA with out any issues.
Good Luck!!!