Welcome to the forum.....
To answer your question about "what powers the solar ready plug"... The response is "It depends on which "Solar Flex package is installed on your trailer".....
If you have the Solar Flex 200, it does not include an inverter to power the outlets. They are "wired to a loop of ROMEX that you can cut to install an automatic transfer inverter"... All that's included in the 200 package is a solar panel, a 15 amp charge controller and the connection fittings on the roof. The dealership may/may not have connected the system to the battery he installed in the trailer.
If you have the Solar Flex 400i package, it comes with two solar panels on the roof, a 30 amp charge controller, an inverter to power the "solar ready outlets" located throughout the trailer and a "bluetooth shunt" to monitor battery status. BATTERIES ARE NOT INCLUDED FROM KEYSTONE, so the dealer installed batteries are "his choice" unless you negotiated the brand/type to be installed.
You can read a more detailed description of the available Solar Flex packages on the Keystone website here:
https://www.keystonerv.com/solarflex_landing
Short answer to your question: Unless you or the dealer installed an inverter on your trailer, those "solar ready outlets" will only work when you're connected to shore power or a generator.