Flex Seal repair on grey tank.

If the crack is in the bottom, in an area where the tank is going to flex a lot, and not well supported chances are slim an adhesive will last too long as the tank fills and sags.
You may want to look at adding a support under the tank to take some stress off of it.

I tried some Flex Seal spray to try and seal a small leak in a corner gutter. Awful stuff, never worked at all, just foamed and did squat. I've used Goop on a lot of stuff and it is pretty good stuff. I don't think JB Weld would work as well because it has no "give" when set.

Thank you all for the input.

I used some tank repair from camping world twice. Crack at corner, stuff held up good. Crack where drain pipe connects not so good. If doing over I would pull tank so I could "prep" bad spot better. Might of help up better?

We used on a crack in the tank of our commode in our S&B several years ago. We put 3 coats on it, letting it dry the recommended time specified. It lasted for about 2 years before it began leaking again. I wanted to dry, spray and let it work for another two years, but DH decided he would rather just get a new commode.

I've used goop on everything from my slide seal to shoes to motorcycle seats. Most of the time it's sold out at HD near me. 600 is often sold as a fabric mender and seems similar to goop.

If the tank is black abs I've also repaired with a kit found online. People use abs shavings and mixes that you can find on YouTube

Goops contribution to mankind is right in there with weedeaters, bungee cords and chainsaws.

Not on a grey tank but I've had to use it on my shower stall floor.
It held up fine for the last 2-1/2 years we had the camper. The tape is very similar to eternabond. I had to use it in a pinch and was surprised at its durability.
I drilled a 1/8" hole at each end of the crack then cleaned/prepped the floor with naphtha then cut a piece of flex seal a few inches to overlap the crack and it held up great.
Very sticky to work with if you handle it a lot.
I used the flex seal paint over the tape to blend the tape in with the bottom of the stall. Stuff worked good also. "As seen on TV" product made me squirm a little, but, it worked well.
Good luck,
Rick

Unless it is a replica of the San Andreas fault, GOOP adhesive is as good a repair as any. The crack need to be dry. Place the nozzle of the tube hard against the crack, squeeze then follow the crack. Repairs have outlived my ability to keep track of them years later.

This stuff stretches like a rubber band and adheres tighter than a Time Share salesman. Scuffing up the edge area alongside the crack with 80 grit makes it stick even tighter. Want decorative results? Paint over top with RustOLeum black bed paint.

Unfortunately I gave it a try on other projects around the house....my opinion: junk and hype.

Might look at something like JB Weld for the repair on your grey tank..
Good luck with it.