most likely the space between the fuel tank and body has filled up with crud over the years; stays damp for long periods and accelerates the rusting process.
That'll be it, then when damp the soundproofing doesn't help much either.
Patrick you should also check the floor above the cross-member, just below that area, as the flanges fold up to the sloping section. The very corners above the jacking points too.
https://club8090.co.uk/wiki/Ar ... _door_side
Rake around through the 1" holes in the end of the rear cross-member that are just fwd of the outboard traling arm mount bolts. If a lot of crud is in there, then suspect the ends of the cross member are thinnned severely.
https://club8090.co.uk/wiki/Ar ... oss-member
But lets think +ve, it could just be the seat belt anchor area, although its a warning to clean out all the sills, cross-members and chassis members (rear partic)
At 20 years +, it's probably a good time to punch decent sized inspection/access holes in a lot more places to keep them raked out and treated. Presonally I think just squirting loads a waxoyl is NOT the solution, it just prevents getting most of the crap out. Drying with hot air and raking then vacuuming (repeating ad nauseam)
before treating is my belt'n'braces approach, but not everyone agrees
Tank straps are available from Germany (expensive) or in fact not really that difficult to make up - retain the end fittings from old ones helpa a lot.. Thin s/s strip is good.
PS. It's the fuel pump you should worry about, not the tank falling out.. check pump for corrosion of main body