You don't say where you are or much about the conditions under which it appeared. Is the unit stored, are you using it, living in it.
When I store mine all four roof vents are left open. They also all have covers so they can be left open and no rain get in. There was only once when I saw some condensation, one year when we stayed home in the winter and the temp was going above and below freezing, on the head mirror and that was gone the next day. ALL doors and cabinets are left open.
We store over the summer as we travel in winter usually. There is still cold, rainy, wet weather at both ends of the trip and start of storage yet no trouble.
Take a serious look at ventilation and covers for the roof vents, all of them. Then open them. That will promote circulation as even small breezes will set up high and low pressure differentials across different vents and move air.
The manual vents are foolproof. The Fantastics, I open, and then go outside and shut off the battery disconnect switch so there is no power for them to close and they don't use power while the trailer is stored.
Even with a dehumidifier, so you have 120V power????, you may need one or two fans to force air circulation inside other wise the dehum. will remove moisture from the air in the immediate area but there will be many dead spots where air will stagnate and allow humidity to stay high.
If you can get the roof vents open you may not need the dehum. and fans.
As far as removing any mildew, bleach will kill it. So will Hydrogen Peroxide. Just wet a rag with them, one or the other, and wipe carefully and then wipe down with a damp rag as the chemical may want to do a job on the wood finish. The peroxide is milder. WEAR GLOVES.