Just to play devil's advocate for a second...
Mold needs certain conditions to exist, primarily being moisture, temperature, and a nutrient source (such nutrients are indeed present in many RV construction materials). The presence of mold does not automatically indicate the presence (or past presence) of a leak.
Example: We left a piece of clothing in our camper a while back. It was in the enclosed cabinets above the master bed. We don't know if the clothing was damp, dirty, etc., but a few weeks later we found it and it had mold growth. No air circulation in this compartment. We know this was the result of something other than a leaking rig.
None of these units are air tight. Any well constructed unit (just like a house) is designed to breath. Those air-volume tests that RV manufacturers advertise are just a gimmick. We have found the best thing to do is to keep a dehumidifier running full-time in our rig. It doesn't run all the time, just according to the humidity setting we put it on. We also have the luxury of leaving it connected to power full-time, which is something that many folks can't do. With the humidifier running, coupled with the AC unit set on about 80, and leaving the doors to compartments open (to increase air circulation), we've been able to maintain a very mold-free rig.
Of course, in the OP's case, it could also just be a leaky roof LOL