EZPass is a consortium of sorts. Any member state's transponder will work throughout all the covered roads in all the member states. Get a pass from whichever state is most convenient or offers you the best deal. (Illinois iPass is one of the members, even though it doesn't call itself an EZPass.)
New York might be a reasonable one to consider, particularly if you foresee traversing the NY thruway frequently. If you get it from the thruway commission, there's no periodic fee (as I understand it), and they have a special motorhome/RV version that charges car rates for motorhomes on the NY thruway.
It's still generally possible to get by with cash, just rather less convenient and possibly a little more expensive in some places.