That's a common area. Many things can contribute so I'll just tick them. Check them all. It may well be more than one thing, but let's hope not.
1. Tank, fittings, filler neck, water lines.
2. waist band
3. rear window
4. tail light
screws through the hull
5. spare tire/table "block".
The rear benches can be removed easily enough. Just a few screws holding them in. Remove tank, filler, lines, etc.
Open up the rat fur. Our 2002 was seamed in the center and we just pulled back the requiste side like a swagged back curtain. Ditto with the bubble wrap insulation.
Now, look for a dark stain trail. Water coming down from the window or tail light
or spare tire block of wood, or from a belly band leak will often leave a tel tale trail of dark "tracking"
. If all this area is clean and shows no black tracks or trails, it is possible that your suspicion of the tank being the sole culprit may be right. But while the area is opened up surgically, test everything. A nice heavy rain works best. A strong flash light
will help you detect any trickling water. There is no sense in tackling the floor cut out and replacement until the leak(s) have been identified and repaired with 100% certainty.