In most such systems the rear a/c will receive power directly when the genset is running, and the manual select switch stays set on "front". Sometimes the switching is smart enough that the manual can be on front or back and the genset power will flow to both.
As others have said, your genset produces two outputs, one 30A that is the equivalent of the shore cord and powers the main power panel, and a secondary 20A that feeds into a dedicated a/c circuit. First check the breakers on the genset itself - the 20A breaker may have tripped. If that's not it, I'm guessing the relay that activates that dedicated alternate circuit to the rear a/c has failed. It is part of the automatic power transfer mechanism, but I don't know enough about your coach to pinpoint where to find it. Start looking where the shore power cord and genset power cord meet up