From what I understand, each state's E-ZPass rules might be slightly different.
In PA, Motorhomes need to sign up for a commercial account. Here's a link to do so...
Additionally, if your account is setup in PA, you will need multiple transponders. I have one for my MH by itself, one for my Toad by itself, and one for my MH pulling my Toad. I was able to register multiple passenger vehicles for the Toad transponder, so that I can use it in the Toad or my wife's vehicle.
As a side note, I have not had the opportunity to use my transponders outside of PA yet and I'm not sure how non-PA transponders work in PA.