In a bind with vehicle registration...help appreciated!

Welp, I've really done it this time. I registered as a weeklong rider months ago, but forgot to add the RV we're renting as a registered vehicle. I tried to get the license # from the renter a few weeks ago, he forgot to send it, and I completely forgot that I still needed to register it before April 1.

Now that the registration date has passed, I can't get a vehicle pass. I've already contacted Ragbrai and their best suggestion is to get one at the EXPO, but it's first come first serve, and we're traveling from Kansas City, so I don't think I can get there without driving up the day before (which adds the logistical issue of where to park and how to retrieve my car).

Does anyone have any suggestions, or even better, a vehicle pass they'd be willing to sell me? Thanks!