We bought in IL EZ pass and registered four vehicles to it for the summer. Our truck camper (registered in SD), my son's car (MICHIGAN license plate), our friends came in from Montana pulling a dual axle trailer from WY and I registered (online) their Montana truck their friend's WY trailer on our device.
The device was in our truck yet all those vehicles came and went thru IL, IN, and MI for two months without any problems.
And we only set our device on the dash. We never mounted it to the windshield or anything else because we knew we'd be canceling it and returning the device when we left.
We did have a printed copy of the EZ pass registrations in those glove boxes but no one ever asked to see them. Everyone drove thru the unmanned gates.
And to get those registrations I just did a print screen of the online account showing those vehicles and plates were registered to our EZ pass.
Kathy