Ok - I bit the big one and called the customer service center. We have to go to Harrisburg and get a commercial transponder. Thankfully we will be heading to Lancaster tomorrow for 3 days so we will head to Harrisburg on one of those days.
I'm sure that we could have fudged it and registered the one I bought yesterday under our truck then tried to put it in the motor home but we really don't like taking chances on being pulled over for some violation. I have no idea what those might cost!
So the one we have goes back tomorrow and we have a bonus trip planned now.
I did check into the Illinois ezpass but again the issue is the waiting the 2 weeks to get it. We aren't stationary anywhere that long (which is a good thing!)
Thanks to all who answered. Hopefully this was a help to our fellow northwesterners who spoke up above.