I had the exact same issue on my 196. In my case, I have a tight-fit waterline cover. I also use 303 after every use. The stains only appeared in areas the cover touched vinyl and after leaving the boat covered for more than 2 days in direct sunlight/heat. Once I discovered the issue, the stains appeared to fade away throughout the next couple of days while using the boat and leaving uncovered (parked in the garage). My solution has been this: I use white bath towels to lay across the back hatches, back seat, gunwale pad, as well as the motor box (essentially anywhere the cover may come into contact with the vinyl) and never park the boat out of the shade while covered. This has seemed to resolve the issue. Going forward, I will continue using the towels, and I will be keeping a small tub of DampRid in the boat in the case it gets covered before fully drying out.
Hopefully your vinyls will go back to normal with enough direct sun exposure and 303.