Originally Posted by OswegoLion
Water is running under the black tightener connecting to silver threaded short pipe.
This leak is not coming from the top of the drain at its flange - it is coming from where the rubber seal above the nut you have pictured is not sealed properly to the bottom of the sink and/or where it should seal at the tailpiece threads.
This is typical with cheap plastic lavatory drains, and the way to deal with it is to loosen the nut, slide the bottom gasket down so you can see about ten threads and put pipe dope on the top of the seal and on the threads you just exposed. Now, tighten the bottom nut back up. Problem solved.
The thing about plastic drains like this is, you have to get the bottom nut tight enough to compress the rubber seal around the threads of the tailpiece. Many of these cheap drains can't be tightened that much without splitting the top flange, so you have to be careful to not overtighten the nut, but get it tight enough to make the bottom seal.
This problem is a common Monday service call for me. Homeowners spend their entire weekend installing a lavatory faucet or two which they bought at Home Depot or Lowe's with the junk plastic pop up assembly, and they just cant get it to stop leaking (right where yours is leaking). I show up and have the problem fixed in a couple of minutes only to be criticized for how much I charged them for something that took me so little time.
Edit: I realized that I overlooked one important factor - many RV lavatories don't have overflows like residential lavatories. Without an overflow, the tailpiece should not have slotted drain holes in the top portion. If this is the case, just resealing the flange at the bowl should do the trick.