With NJ, one can have 4 transponders per (non-commercial) account. I called their 1-800 number and let them set me up with a classification that lets my motorhome or pickup tow (something).
If, for instance I rent a trailer, I go on-line to my account and add the license plate of the trailer...then when done with it, simply remove that plate.
As far as my state and other states, that i travel thru, I use the EZPass lanes all the time. Never a problem.
((Edited to say: when the motorhome is pulling the "toad", remove the transponder from the "toad" and put it in that plastic bag it came in...then put that in the glove compartment or you will be charge fully...for the motorhome AND the "toad". ))