Holly-
First off, welcome to winnieowners!
Second, you should add a signature to your user profile. It usually includes the year, make and model of your coach, but can include other information. A signature is appended to each post. To create/edit a signature, click on the "User CP" link in the orange bar at the top, then click on "Edit Signature."
Re: Trailer electric brake controller. Your coach is new enough that it should be a "plug-and-play" installation. I believe your Solei is built on a Freightliner diesel chassis. If so, these links should help:
Installation Supplement for Brake Controller Pigtail for Freightliner Chassis (built after December 2002)
Tekonsha 90195 P3 Electronic Brake Control for 2019 Phaeton
Tekonsha 3027-P Wiring Adapter for Freightliner
Please confirm all this info for yourself- don't take my word for it.