A couple of weeks ago I got a MA EZ pass (2 actually - 1 for the MH and 1 for the toad). No charge for either, just $40 into the account, and allowing them to replenish it when it runs low.
I was told that when using it in other states, I would have to go to a manned toll booth and have them add the extra axles on my EZ pass when towing the car. I have since found out that you can have a transponder set up to include the extra axles to eliminate having to have the toll taker manually input the extra axle/s. I'm going to call them to see if one of my transponders can be set up to include the extra axles.
I assume there are states that have eliminated toll takers, so I'm not sure what would happen if I drove through a toll plaza without having my extra axles manually added in my current situation.
A bigger problem I've had is that in 3 of the toll booths I've gone through my transponder wasn't read. I put it in the middle of the windshield, maybe 8" up from the dash. One toll taker told me it's not reading because of the cabover overhang. I'll try putting it lower on the windshield.
I put my Sun Pass transponder low on the left side of the windshield and have never had a problem with it being read.