So I don't believe now that that stuff is plumber's putty. It's too tacky and sticky still after at least 6 years (and 6 mos. sitting apart like that
The closest guess I have is "Gray Fresh Water Mastic" but I can't really find any photos of the stuff. The box hardware stores are clueless of course.
I called Thetford... waiting to hear back, the initial response was "we don't know what you're talking about, send a photo"; despite having a photo onhand they still insisted on calling me back instead. Hopefully with an answer before the weekend...