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Hello I will help you with your question,
I am surprised they didn't offer to do it at the RV sale unless this is a private sale. Most RV shops will be happy to install the wiring you need.
You have provision for a four wire flat connector already wired in but with the 7 pin you supply 12 volts to the trailer as well as have the braking function in addition to the vehicle lights.
Any RV, trailer or even UHaul shop (last choice) can install the wiring or you can have an independent certified general service shop do the work. It is not hard but it does require access to the service information for your 2015 Subaru so they can make the correct connections.
Please let me know how I can help you with this