2012 keystone sprinter has two grey water pulls and one black. Which is black one

You can find free shop manuals for 2012 Keystone Outback 322FRL online on the manufacturer's website. You can also search for the manual on eBay or Craigslist, or contact the manufacturer directly if you are unable to find the manual online.

This is not just for your camper, but general troubleshooting.
Drain the water from the belly pan. Hook up water supply to shore water source. With water source turned on, allow some time, 1-4 hours, and check if water has returned to belly pan. If water is present then the problem is with the fresh water system. If no water, either it is a small/slow leak or the problem is not with the fresh water system. Next test the grey water tank by pouring some red food color into the shower or sink drain. Flush it into tank with water. Again, wait for some time, maybe over night, and then check for red water in the belly pan. It may take longer for the leak to appear as the grey water tank does not have pressure. If no water, check the black water tank with green food coloring and same technique. If there is colored water, red or green, in the belly pan, then the leak is with associated with the grey or black water tanks. If no water is present after these tests, then the leak may be very small and it may take days or weeks to have water re-appear. In any event, clear water is fresh water system, red water is grey water system, and green water is black water system. With any of them, checking the tanks themselves and all pipes and connectors to/from should find the leak source. The last possibility is that water is entering the belly pan from a roof or vent leak. Check to see if water returns to belly pan after a good rain. If so, the water may be clear or slightly dirty. If it is a roof leak, I have found the best repair is to use Peel and Seal roll roofing It is light weight laminated aluminum and self sticking. Cutting and applying narrow strips 4-6" wide, will seal any caulked area. It costs $95 (3' x 33') at Lowes. A 30" motor home can be completely covered with three (3) rolls, in half a day. I have re-topped 5 motor homes (one 7 years ago) and no more leaks on any of them. The product was developed to seal the roofing joints of mobile homes. Topping should last 15+ years.

Pull bottom plug out of hot water heater tank let all water drain. Then open all water tank drain valves and be sure your RV is tilted so it will drain towards the valve. Go in RV access water heater valves put it in bypass so no antifreeze can enter water heater tank. Pump RV antifreeze with hose provided into all water lines throughout RV by turning on your main water pump after you have stuck the bypass hose in a new and full antifreeze bottle, then flush toilet until red antifreeze comes out. Open all faucets one at a time until red antifreeze comes out. I use 3 gal but some only use one, the more the better it’s cheap at Walmart. Don’t forget to run outside shower until red antifreeze comes out also, very important or it will crack in the cold. Finally make sure black and grey tanks are empty and flushed at your last camp trip before winterizing. Pour antifreeze in toilet and all drains making sure first that black and grey tank shut off valves are pushed in and closed. Get a quality cover and store RV, it makes a huge difference on protecting roof and roofs lap sealant! All RV antifreeze is non toxic and you just drain it all at your first camp stop after winter! Note: don’t forget to put water heater ionizer drain plug back in wrapped with new white Teflon tape and take water heater valves out of bypass! Draw a diagram of regular mode and winter bypass mode so you don’t forget especially hot and cold water heater blender valve between the two MUST be shut off after you unwinterize
or you will cool your hot water unwanted and have no hot water to use! Happy Camping folks!

Best to contact Keystone's Customer Service (as no one one here has any manuals).

There is usually a drain valve for each tank could be under trailer in middle some where

That should be easy to locate. Outside, down low. Usually black, 3" diameter. Has a cap over it (or should). The unit will have 2 pipes running into it. One usually 1 1/2" or 2" for the grey water and a bigger 3" for black water. Made out of PVC plumbing pipe, just like in a house.

On some later model Montana 5th wheels, there are actually three holding tanks which will eventually require draining. (two grey tanks, 1 for the kitchen sink(s) and 1 for the bath tub & bathroom sink, and one black tank, obviously the toilet...) The two most used drain valves (1 black & 1 grey) are together in the docking station area where the city water supply is hooked up, the other (kitchen sink) is at the left rear of the coach behind the farthest rear wheel along the frame. (That is where "Keystone" installed the grey tank for the galley, or kitchen if you prefer.)
If certain it is the black tank which won't dump when the black tank valve is pulled, there is a restriction before the valve (or the valve itself is not opening hence a broken cable to the valve?) One might try "snaking" up into the sewer line, measuring the distance from the outlet to the valve location, assuring it is either open or closed. Once that has been determined, you will at that point have to remove the sewer line at the valve for repair. Be sure to wear latex gloves and eye protection and have a drain pan or bucket handy for spillage which can be collected and dumped down your home toilet or outside sewer clean out if you know its location.

check with the dealer they can help you find this

Contact your local retailer directly or online here:
http://www.keystonerv.com/contact-us