Interior gas tank fumes

[Beatcoaster]

I don't smell them often, but every once in a while I get a good whif of gas and was wondering if anyone has found the source of them? Is it the sending unit cap letting them out? I have thought about globbing on the silicone around it to see if that would cure the issue but just wondered if anyone has found the cause, since everything else is sealed an/or vented outside. Until I get the cash to put in a suburban tank I don't have too many options other than dealing with the death trap behind my seat. Any ideas?