I've never used soap, but it's a method often recommended.
A few months ago I met a South African in Namibia driving a Landrover back home after a tour around south-western Africa.
Through rock damage, he'd got quite a bad leak in his petrol tank a few weeks earlier and proudly showed me the repair, done with soap.
"But it's even better if you mix sugar with the soap, but not too much," he told me.
Sugar reacts with petrol, usually detrimentally, but he claimed a little added to the soap helped to stop the soap drying out and caused it to stick better to the steel. He was an experienced traveller in Africa and expected his repair to last at least all the way home, another week or so.
Maybe worth a try.
Or can you get "Araldite?" Or any other two-part epoxy adhesive in tubes.
A year ago my son-in-law mended a leak in his petrol tank with that, and it's still all fine.
Use sandpaper or a file to make sure the area around the leak is perfectly clean, and that any paint is firmly attached to the metal still (or better, remove the paint if you can). Make sure you get rid of ALL rust if there is any around the leak.
Spread a small blob of Araldite over the leak and let harden completely.
Then a slightly larger blob over that, spread more widely, let harden completely again.
Repeat a third time.
And don't forget, if your tank is leaking at all, it becomes quite a fire risk. So take extra care everywhere, specially where you park it.