It is important to make a distinction between a hull that lists when at rest and one that lists under power. As the OP indicated, it seems the hull lists at rest. Since the hull has been listing more now than a while back, it is safe to assume that there is added weight towards starboard .
The most likely cause of extra weight is water of course. That could be wet stringers, wet hull, or more likely wet foam. When the foam gets wet and that hull is foamed, it can stay wet for a long time. More importantly, the wet foam in low temps can freeze and expand leading to other structural issues. It is also possible that the mid bilge area has water that doesn't drain aft. That is commonly caused by plugged drain channels. You should be able to see the drain channel from the aft bilge and snake it out. I don't think there is access to the mid bilge area so checking from that side may be difficult.
If water gets in through the anchor locker, it normally drains aft. If the drainage is blocked water will buildup mid ship.
You may be able to remove the panels under the helm and get a look at the foam. I am not sure, You might need to remove the built-ins from the under the helm seats to check. Note that as the hull lists, liquids like gas and bilge water tend to move towards the side of the list. That can make the listing seem worse.