Cant speak for Redbays only.. but the principals are the same, most ingress I have encountered are via deck mounted structure. (take it your Redbay stuff is bonded though) If your gonna blow air up the under deck.. you need to have 5 peeps with soap on hand to see all the air come out
!! 1 wont do . As JW said trunks are a pain too, most of mine have not sealed but they are quite easy to spot so I doubt thats your problem. More than likely its a small deck hatch somewhere,or something similar.. sealed though they are supposed to be.. have you got any of them ? Im always surprised at how small a leak can gather a lot of water, particularly if its in a path of water getting shedded by a console for example..Or its some other fitting you got added perhaps by a third party, who didnt seal it correctly,. or does you fuel tank sweat on the outside a lot ?
Best of luck though as I know how annoying this is