need to patch my black tank

"grate" ?

only 1/4 " hole use a tire pluging kit should work grate

I'd just get a roll of Eternabond. Many hardware stores are now carrying a 2" by 10 foot roll.

Cut out a square, clean the area, stick it right over the hole. Done.

Use Shoe Goo or Goop That's all I use for adhesives anymore. You'll be good to go.

Make sure the black tank is EMPTY before you start looking IN the hole..:W:W...Dennis

Quite some years ago, the black tank on my Travel Trailer got a crack in it. At the time, U-Haul serviced RVs...and taking it to them, they had a "plastic welding kit". Once they found out what kind of plastic the tank was,(the kit included a number of different welding rods) they "welded" the crack (maybe 6 inches long) it with this thing...and never a problem for the 4 years later than I had it.

Watching the guy do it, it didn't look very hard to do at all. 15 minits start to finish.

just FWIW.

I ran a screw into the FW tank on my camper several years ago. Used a hot glue gun to fix it. Let the gun get nice and hot, then shoved the tip into the hole and let it melt the tank plastic. Then pulled it out while squirting a liberal amount of hot glue into the hole. Left a nice big gob on the outside. Hasn't leaked yet.

:):)

Go to any auto supply store and pick up a tube of black windshield adhesive. used it on my gray tank worked like a charm pluged a hole about 7/16" and no leaks

Use the well nut from the link above. To ensure a permanent fix coat the threads with Locktite 242 when installing. Easy fix..assuming, since you got a drill in there, you can also get a screwdriver in.....Dennis

If you have a choice - stainless steel only. Even brass will be eaten by some household cleaners.
With access on both side I would run machine bolt with big washers and gaskets used on toilet tanks.
That will make lifetime fix. All others will be something you might have to worry about again.