Sparky, I don't believe the 2008's had a check valve - my 2007 does not. At the end of the hose, attached to the tank is a plastic screen fitting that gets clogged with fine paper threads. When this clog dries, it is almost impossible to clear the clog with just the pressure from the hose. The filter/fitting needs to be removed and cleaned by hand.
On my rig, the fitting is towards the curb side of the rig, on the long back side of the tank. I have had to clean mine a couple of times. I access the fitting by pulling down the membrane from the curb side about 2-4 feet along the edge of the RV. I can then reach my arms in to remove the hose and fitting without pulling down the entire membrane. BTW, it wasn't clear in your writeup, but to properly flush the black tank, you need to fill the tank up with the blade valve closed (after you have dumped) and then drain the tank. I do this two to three times when flushing.
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