I saw a sign at a service center for RVs that said:
"If you think you don't have a leak, you just haven't found it yet."
In Western Washington, we know a bit about rain and mildew. Most shops will pressurize your unit with a powerful fan running into your top vent, then squirt soapy water all over to see where the bubbles indicate a leak. Simple and fast. I had a leaky top light and TV antenna base. $75 seems cheap to diagnose the mysterious travels of water. The top running light was centered, but the water showed up on one side at the floor!