If the tank has a problem they are fairly easy to drop and you will then have proper access to assess the condition of the tank and all those old rubber parts. It may indeed just be the rubber bits, they were not designed for 30 years use.
Fuel tanks are not that expensive so if it was leaking you may consider it easier to just buy a new one than try to patch a leaking one. The fix to a rare or expensive tank is usually to coat it internally and there are plenty of products on the market which can do it.
BagusJeep lives in Bali.
1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE