In addition to a interior redo, I would also try using 4 or 5 Damp Rid buckets (available at WM and HDepot). We had all types of mold and mildew when we first brought the boat. But we started using the damp rid and it really help reduce the moisture in the boat which in turn, reduces the mold and mildew. I have seen no evidence of new mildew since. We have to empty the water out of them out about once a month. Its kind of surprising how much water they pull right out of the air. They also help the smell as well.
Besides, you can rip your cabin it out every few years. That may start to get expensive.