MFL wrote:
Hey good news, at least you found the leak! It is really hard to get ALL the water out of one of those tanks. If it needs to be dry to repair, I'd pull the trailer up on 1 or 2 2x6s on the drain side, to move water away from the drain.
I would not get in a hurry, and goop it up with the wrong thing. I would call an expert, such as the tank manufacturer, explain, get their thoughts for fix. You may have to get tank info from trailer manufacturer, and they might have some ideas. I have repaired ABS tanks, black tank with good results, but no experience with fresh water tank.
There are a number of RV techs over in the tech forum. You might post a question there, and get a fix idea, from someone with FW tank repair experience.
Jerry
This plan came from a Rv repair guy. Of course he also suggested using a hot glue gun to partially melt the tank and reseal that way. I'm nervous to try that so he suggested the above idea.