I worked all weekend to solve the problem. Thanks for your reply.
Tore the trunk carpet off and found that the gas was leaking from the gas filling cap and the black rubber grommet lost its elasticity (the black piece of rubber around the hole when you pump the gas). So after a fill up on a hot day, gas bubbled over through the cap and leaked down the right wheel well in the trunk soaking the trunk carpet.
After scrubbing it with soap and water and airing it out, it has improved, but I'll have to get that new piece of rubber tomorrow and wait awaile before it gets back to normal.