Grey Tank leaks

Glad you found your leak. My grey leaks from somewhere above the visual line. To get to it I would have to remove the bottom fabric that covers the bottom of the 5er. The tank is not exposed. So I just keep the grey valve open so it doesn't leak.

I would second MFL posts
Or at least try this before dropping the entire tank to fix it

ABS cement, in small can works good for ABS tank repair. It's readily available at most any hardware store, Lowes, Home Depot, Menards. Get a small can of ABS cleaner to use first, then add a thin layer of the black cement to dried surface. Now cut a small patch of fiberglass screen (like in your RV screen door) and press/apply it to the wet ABS cement, let dry. I use 3-5 more thin coats to cover screen, and beyond. Repair will look good, last forever, and is easy to use on a non-level surface. I've never had a failure, using this method.

Cement dries quickly, so have your screen cut, a bit oversize, ready to go.

Jerry

I,m for dropping the tank and plastic welding it, I,ve seen miles of plastic pipe welded works great goes on like melted butter.

I have in the past used fiberglass cloth and rosin to repair a crack in on of my holding tanks. I first drilled a small hole at each end of the crack to keep it from expanding. I sanded the area down cleaned it up and went ahead with the fiberglassing. The down side was I was working against good old gravity. This made appling the rosin very messy. That being said it worked for me.

Drop the tank and clean and dry. Cut a hole in top so you can do repair from inside. Glue tabs to the piece you cut out to keep it from dropping inside. Secure and seal patch back in place.

As mentioned, once you have the tank nice and clean and dry, take it to any shop that can do plastic welding and have them do a complete proper repair. Auto body shops all have the ability as well as plastic fabricators (some may, others won't). Even some mobile RV techs have the welders with them as many RV parts are plastic based.

If you're adventitious, buy a plastic welding kit and give it a go. Who knows you may be able to recoup the cost doing mobile RV poopie tank repairs. :B

Get a plastics shop to weld it. I had a tank repaired by a shop that refurbishes auto bumpers.