Bob,
I assume it is a general odor through out the entire boat, because you did not pin point an area that it is coming from.
First use Clorox 1 part to 3 parts water. Wash everything, cabin sole, bulkheads, liner, etc. Make sure you are well ventilated while doing this. Run Clorox through the holding tank, pump it out. Run Clorox through the bilge and pump it out.
If the mildew smell is in the upholstery you will most likely have to replace it, but try washing or dry cleaning which ever is right for your fabric. Washing does a better job of removing odor, so if you have a choice, wash.
Remember to keep the boat well ventilated and try not to inhale the Clorox.
Before you do any of this, try to sniff out the source. It might only be coming from one place.
If the smell continues after the above treatment...SELL.
Good luck,
Larry