Bleach should kill it then scrub the whole area down with a good antibacterial soap solution and scrub hard.  (Bleach kills it but scrubbing removes it.)  Rinse really well and once dry you'll be needing to put at least one layer of wax down to help seal out the elements.  Compounding first would be optimal.  The wax should prevent this from happening again.

I've only seen this happen after people park under trees, also tree sap can cause this condition.  Regular washing and parking in full sun to dry can kill mildew, but only if it hasn't already set in on the trailer.

Here's to hoping you win!