We have a DE EZPass for each of our 2 vehicles and another for our RV. Never a service charge for any of them. WHen pulling the EZPass equipped toad the toad pass needs to go into a special plastic sleeve to prevent you from being charged twice. When this is done ALL EZPass roads, bridges and tunnels count axles to determine my toll.
Some points:
Our pass works all over the east and southeast, but last year did not work where Florida Sun Pass or Houston, Texas toll roads were charging. We have used it as far west as the toll roads around Chicago.
Some states like Maryland and NY charge different toll rates depending on whether the EZPass was issued by the local toll authority or an out of state authority.
Some bridges and roads will charge more when your RV is pulling a toad than when the same RV and toad go through the same plaza separated. In Maryland's case the increase is $12 more for the Chesapeake Bay Bridge and similarly for the Delaware Memorial Bridge. The Mackinac Bridge in the Michigan upper peninsula does not do this.