It is possible to repair the tank while it is still in the lazerette- but very difficult. I had several holes in the bottom of my tank (1989 28) about the size of pencil erasers. After cutting the bands that held the tank in place, and then sanding the entire surface of the tank and up the sides, I used New Pig Aluminum Repair Putty to fill the holes and cover any spots that looked "soft". Then I used West System G-Flex to cover the entire bottom with a liberal amount of material and also up the sides. With new SS bands and a huge amount of effort the tank was back in place.
If your wondering- I'm 6-2 220 65yrs and yes I did sort of wonder if I could actually get out of the lazerette when I was done.
This "fix" is probably temporary and I will need a new tank eventually and then I will use the bulkhead approach. The repair has held up for 2 years in Minnesota climate.