Based on the info in this thread I stopped at the Oshkosh WI DMV. I brought with me the following three items:
"Certificate of Origin for a Trailer" which I received in the mail from ETI.
Copy of the bill of sale, again provided by ETI in the mailed packet.
Less than 10 minutes later (after writing a check for WI sales tax based on the Bill of Sale) I walked out with my title, registration and license plate. I will be picking up a lottery ticket later today.
Thanks to all my fellow Wisconsinites for contributing to this thread. I'm hoping with providing just these three pieces of info we can avoid confusing DMV employees with the additional paperwork ETI provides and make my experience the norms in this state going forward.