Diesel leak from tank drain

The reason the epoxy did not bond well is probably due to fuel contamination at the bond line. I would drain the tank completely to stop the leak sand the surface or use a wire wheel where you are going to put the adhesive then wipe the area several times with acetone or MEK. I would apply about a .020 thick bond line allow it to get tacky then apply FG cloth that is wet out with the resin over the tacky bond line. The FG cloth will prevent the resin from cracking. The more surface area that is cleaned the better your chances of the adhesive holding. I would use an epoxy adhesive with a longer pot life like 20 to 30 mins.

Our tank must have been made by the same folks. Leak around the drain plug weld, but ours was a little more severe. I couldn't find any other was for a permanent fix except to pull and weld. It was an $800 shop visit but luckily service policy paid.

I recently had a hole in a plastic fuel tank that required removing the engine to get it out. I could just see the hole through the fill neck. JB weld and a 3' of rod with the end flatten into a spoonbill let me flatten the putty out on the inside. So far it's not leaking without removing the engine or replacing the tank.

On a metal tank I'd weld it in place with plenty of diesel in it. Weld in short lengths cooled with water.
Yes, I've put a match out by dipping in diesel.

With variety of epoxy you should find one who bond to steel, or whatever tank material is, but for the bond to catch you need to stop the leak.
Meaning draining the tank.

I can't see another way to fix it properly. The fuel is going to erode away at any putty or other non-metallic substance short of a good, traditional weld. Sorry....