I know most of you guys will be using the NH system, but the Maine system is probably using the latest technology so it counts the axles and determines the class of vehicle you are driving. I'll have to see after the first time I do it!
Looks like it might be state specific.
From
www.maineturnpike.com:
Q: Occasionally, I tow a camper, boat or other trailer. May I still use my E-ZPass when I do?
A: Yes, you may use your Maine Turnpike E-ZPass tag to pay tolls on the Maine Turnpike when you tow a trailer. Maine's new E-ZPass system has the ability to identify the size and class of your vehicle as it travels through the toll plaza. The system then deducts the appropriate toll from your prepaid balance. However, before you use your Maine Turnpike E-ZPass when towing a trailer on other E-ZPass facilities, be sure to check with those other E-ZPass agencies.
BT