When I read this originally, I thought you were talking about leakage near the grey/black drains. Now I see you were talking about the fresh tank drain. It seems odd to me that this would only occur when on city water (even with the tank empty).
So just to clarify: if the fresh tank has water in it and you use the pump, no leakage is observed and the pump doesn't run unless you are using the water? But if attached to city water, leakage is observed even with the fresh tank empty?
As to your coroplast question, I too would like to hear from anyone who has opened up the coroplast. I was poking around with an endoscope and discovered the front-curbside corner of my fresh water tank is sitting on top of a short L-bracket that was intended to keep it from sliding toward the curbside. I likely have to drop the coroplast to fix this. Unfortunately the sides of the coroplast are tucked into the C-channel frame members and "fastened/sealed" there by expanding foam from the top. I can't see how to reseal it well after the repair from below. It may be best to just cut a three-sided rectangular access hole/flap in the coroplast (uncut side toward the front of the trailer) and then tape it shut afterward.
2021b Micro Minnie 2100BH
2017 Ford F-350 SRW diesel
Previously: Grand Design Reflection 28BH