Houston...We have re-established service! After many, many hours of "what-if's", probability analysis (not really) and many great suggestions by everyone here, we have an operational black water valve without having to notify the EPA or having to post incriminating video on Youtube (sorry everyone who asked for video) :thumbsup .
I was going to wait a couple days before attempting to resolve the situation but decided the long, slow rains today would be beneficial in helping to eliminate any evidence should there be any. So I moved the Bus outside (stored inside), strung out the garden hose, slipped on the newly purchased long rubber gloves, connected the portable tank...and took my last breath of fresh air. Since the discharge angles down immediately after the valve I decided to drill a hole in the discharge elbow in a position that would allow insertion of a long screwdriver to reach the gate blade. The hole was drilled at the diameter that I could later tap 1/4"NPT and install a pipe plug when done.
I inserted the screwdriver against the gate blade and pulled on the handle just to confirm the valve was stuck...but I could feel it move! I pulled the valve handle full open and could feel the screwdriver slide along the gate blade until it was fully open! Great - it's not stuck closed! Wait...nothing is coming out...pause...prod around with the inserted screwdriver...hear a few dribbles, then a little more...uh ho - realization that I may have dislodge whatever was plugging the discharge and it might just coming gushing. Quickly closed the valve then slowly cycled it back open. A few more dribbles, then finally a decent flow!
Fortunately there was only about 15 gallons in the tank so I didn't run the risk of over filling my portable tank. Did a major spray in the tank to dislodge more junk - I suspect its been building up there before I acquired the motorhome earlier this year. So I'll do some more cleaning on it over the next several days and probably replace the valve anyway.
Such a relief and nice to breathe fresh air! Thanks for the suggestions and the link!