Thanks Jeff. If this doesn't fix it, it's back to OEM for me. Since my sending unit seal seems to be holding, I figured it was worth a chance. There's one type of RTV, 16b, that doesn't specifically say not to expose it to gasoline. It's what the guy at my local NAPA recommended last time, and what I used last summer, and it still looks the same as the day I put it on.
So I sealed gas in attachment this morning. I filled the top two threads with RTV, and then layed a bead around the top, screwed it in tight and put some more RTV.
I'll let it cure for a day or two before I fill it up and drive on some bumps. I'll post up the results here, either as a successful repair or a cautionary tale, depending on how it turns out.