When I worked for Kenworth, we had a product we used to seal cabs before the interior was put in. It was called sikaflex. It would seal and glue anything together you wanted bonded. Wood, Metal, Plastic, didnt matter. Now before you run out and try to find it, its an industrial use product. Cant really get it as a consumer. As much as id like to say it is a fixall, Im not going to.
If the tubs leaking, its time for a new tub.