by dinotoad » Fri Feb 11, 2011 6:05 pm
This sounds like a travel trailer rather than a mobile home.
I had the exact problem with my motorhome. The leak was at the rubber connector that is at the base of the black tank. Buy a new rubber connector, in case you need this. In my case I could loosen the connector, work it back on, and then tighten it to fix the leak. I suspect the white stuff is teflon pipe dope rather than plumbers putty. The leak was caused by the lack of support for the black drain pipe. I added support made from plumbers tape (pipe hangers) and have no trouble since.
Your's may be more difficult, but if it is a travel trailer, then you can take apart the black valve to access it. It should have 4 bolts holding it together. Plan on replacing the seals in the valve while it is apart.