Check for leaks
. Check floors for rot, under front and rear windows
and under frig. vent. Bad floors will give a preistant musty odor that won't go away. Mold can build up in closed spaces, cabinets and under the benches just from condensation if kept closed while in storage.
For clean up and treatment, I use Mold Away from the big box stores. Then get the humidity down with a dehumidifier. Stay away from Fabreeze it only covers up the problem and becomes a problem itself. Vinegar, baking soda and a good carpet cleaner and vaccum will freshen thing up. An Ozone generator
can also get rid of odors.
Usually the musty odor is not the problem it is a result of a problem.