Hi,
I think the first step in checking this out is to evaluate the area of the deck around the mast. Have you thumped around to see if there are sound changes as you get near the base? Do note, that on many (if not all) of the Ericsons, the area of the deck where the mast is stepped is plywood core instead of the balsa core found in other areas of the deck and cabin top - so some sound change is to be expected We're looking for the hollow dull thud of moisture damage instead of the sharp rap tap of a nice solid area. Is there any sign of water weeeping from down below around the mast or the cracks you mention?
If it seems that you do have moisture in the area, the next step is to figure out how bad. It would be very convenient if you could track someone down with a moisture meter who could check this out for you without making you pay for a survey. You could go below the mast and drill a small e.g. 1/8 inch hole through the headliner and into the core and see if the bit comes out wet and perhaps water may even weep.
If it turns out the core is saturated and delamination has set in, you will need to repair the deck in this area. This is really not that hard if done carefully, but one step at a time.
Looking at the link Jeff posted, is there a fiberglass box/beam spanning the two bulkheads and providing support for the mast?
-David
Independence 31
Emerald