If there is a musty smell then I would assume there is a water leak somewhere that has gone unnoticed. I would not assume it was just because it was sitting closed up. With that said...
You really don't need a professional steam cleaner. I used them on my MH and ended up doing a better job myself with a Hoover spin brush carpet cleaner.
Where I really made a huge improvement on my current 5'ver was after shampooing the carpet I went over it again with hot water, but this time had added 5oz of bleach in the gallon of water. This worked wonders.