Brusac
One thing that helped me find a similiar leak was to fill the boat up with air a little higher than full...then cover the floor with water from a hose or in the lake, etc. If the air is escaping from the seams the bubble stream will show itself. You might also sit on the tubes at the time to increase the pressure out of the source...you could mix the dawn soap in this water also, but the water itself should allow the bubble stream to be detected.
(I mean it is a raft , after all, and designed to have water in the boat on the floor, and all that) I also , one hot day, inflated the boat on the lake and got some goggles and went under water to inspect the boat and lo and behold..there was another of those bubble streams!
As a last resort, there is a product called "Bixler's Tube seal,designed especially for inflatable boats which acts like "green slime" Tube repair for tires.It coats the air leak, including inter- tube leaking(between the tubes at the place they are joined together) from the inside.
The company states that the product is unlike green slime in that it dries inside the tube, once cured , so that the inflation and deflation process is not affected. Green Slime stays a "gooey Latex " consistancy in the tire, as the tire is not deflated and the centrifugal force keeps the slime fed into the hole, plugging it, Check them out, they also sell to " WEST MARINE" under that company logo. A little spendy..but comes with good reviews.
One other thought, the green slime is about 6 bucks at walmart...I am wondering if a coating of that ,on your boston valves, each time you inflate would stop the leak, if that is where you are pretty sure it is coming from.
Just wipe it off when you are done for the day and repeat as necessary.......might work.