With our Elkmont 26 we hook up our Sewer Solutions unit
http://www.sewersolution.com/ to the grey tank outlet and leave it open for the duration of our stay.
On leaving we close the valve on the empty grey tank and connect Sewer Solutions to the black tank. Sewer Solutions uses a high pressure water jet (15 psi minimum) to totally macerate all paper and solids as the tank is drained. Then you rotate that high pressure jet to shoot the water back into the tank for thorough rinsing. After a couple of flushings the water runs clear.
The unit is available at Camping World for the President's Club price of $129.99--well worth it for us.