Your problem may be EZPass. I often travel in my MH towing a car from New Jersey to Pennsylvania via I-78. If I use EZPass when crossing the Delaware River I am charged $13 (4 axle commercial rate). If I pay cash it costs $6.50 ($5.50 for the MH and $1 for the car). The same is true when you cross the Delaware River on I-80 at the Delaware Water Gap. The Delaware Memorial Bridge is south Jersey is different since it is run by a different agency.
My suggestion, call the agency to find out the proper toll, or pay cash next time and compare that to what you were charged on EZPass. You might find cash is cheaper.