You might try adding "Geo Method" products (Calgon Water Softener, Tide or New Dawn detergent, and laundry bleach), and hot water, if the black tank is not very full (i.e., almost empty). After an overnight soaking, whatever is causing the valve to stick may have dissolved or softened.
Link to the "Geo Method": https://sites.google.com/site/cbruni/
Alternately (and I'm not sure this is a good idea), you could put the drain cover back on and open the gray water valve to flood the back side of the black water tank valve with water. However, you should think this process through thoroughly and make sure the surrounding area (driveway, drain area) is OK for the small amount of waste water that will run out when you remove the cap, as this could be really nasty.
By the way, after allowing this to soak and BEFORE removing the cap and reattaching the sewer hose again, make sure you close all valves. Otherwise, you'll probably be back on the forums to verbally thrash me for coming up with this hair-brained idea.
Normally, this valve is easy to pull out, so it may take some coaxing if it's stuck. Just be gentle and pull a little, then push a little (back in), then repeat. Hopefully, it will loosen and start moving back and forth a little more with each cycle. Resist the urge to strong-arm the valve and yank, as you may end up holding the T-handle while looking at the valve still closed, with no way to open it. The next step after this is bad, bad, bad! This involves disassembling and removing the entire valve assembly, while the black tank is full. I think you can guess what comes next...