Hole in grey water tank

This depends on where the hole is.

They make a bolt with two washers, the washers are cork backed, if you can reach both sides this is a fantastic drilled hole/puncture repair.

Option two has an oval washer on the bolt head end,, Now this is tricky but you can manuavure the oval through the hole, then position it and the round (larger) washer remains outside, Tighten the nut down and it's a one-side repair.. Been close to 50 years since I've seen em though.

Get a set of HORST miricle probes. The Ground (common) Probe can be put in the bottom o the tank if you like.. And will do what you want,, Bigger hole though I think.

Other ideas present as well, Can you drill the pipe about 1/2 inch above the valve? If you drill there you can simply put a sheet of rubber gasket material (Any auto store) and a hose clamp over it, or a pipe repair clamp (Home Improvement Stores) this might be your best choice.

GOOP works well for small things, Do not know how well it will work in this,, But I use it or sealing lots of holes in things.

Are you removing the entire tank or just the valve?

Drive it on the lawn and drain it. I wouldn't drill a hole in a grey tank. EVER.

When a hole that size was accidently drilled in my fresh water tank the repair shop used a "sensor pad" to seal it. This pad is usually used to sense the level of the tank. In this case it was used as a seal and the wires which extend from it were not connected to anything. It worked perfectly.

just remove the valve and stand back, as long as it is just grey water.
bumpy

I think you're making trouble for yourself by drilling.
Why not just drain the tank good and then drive onto a slope or blocks to keep the "contents" away from the valve?
Or just put a kitty litter tray down and catch the "contents?" I'd do that with even a Black Tank before I'd drill a hole in it.

It's gray water, get a bucket and unbolt the bad valve and the tank will drain.