You've named the things that could be draining into the port side 1) fresh water tank leak 2) condensation line from AC might have a crack, or loose 'connection' along the way (you can check both of these by lifting mattress and removing panel access board) 3) bilge pump discharge hose leaking on a mating connection along the way, or loose at discharge and water 'running' back along hose.
-- Are there any 'operations' that you have going when you see the water coming in? Showering, pump out, raining outside.
Since, short of ripping up the deck, there are no 'build in' access plates at the base of the stairs/inboard of the hanging locker, your best bet might would be to get a borescope (they make some fairly inexpensive, fair quality ones that plug into smart phones, or even a stand alone one) and feed it into void from just above the shower sump. If there's not a lot of water coming in could be tough to 'catch' the culprit.
Changes in L'Attitudes
1999 Maxum 4100 SCA