Don’t know your background but you would be best served to do everything possible yourself.
As for the leak, chase it down and get specific. It probably is an easy fix but you need to find it. Should you not be able to figure out the repair you could get a lot of opinions here.
As for winterizing, you need to be able to do that yourself, too. Your owners manual probably has a procedure. Many are available on the net.
For professional help don’t get the Mercedes part serviced at an RV service center. Chase down a Mercedes Benz service center or someone who specializes in Mercedes Benz.
Maybe a local here will give a recommendation for an RV service center for the coach part.
Winterize it yourself.