ok, for those of you that seem to think I've not checked the most obvious:
Yes, of course I tried filling it with the valve closed, as if I needed to since it was overflowing my kitchen sink, then tried emptying it, filling it(with hot water), emptying it, filling it/emptying it. All with the valve closed then opened to empty.
I had a full tank by the gauge and by water backing up in my sink. Full I think is the most obvious answer here. So, I used my portable dump tank(note-25 gallons) to empty it(completely- 40 gallons) because I wanted to allow it the freedom of gushing from the 4" opening of the gray tank opening on the RV rather than the garden hose that it normally would just in case it was going to dispel something large. I wanted the power of the gush.
So, when I drained it till no more was coming out... it filled but maybe a little more than half of the 25 gallon portable dump tank from my 40 gallon gray tank. Approx 12-15 gallons.
I'm hoping I've convince even the most confident doubter that my 40 gallon gray tank has some issue beyond the gauge being bad, a plugged p trap.
Sorry, I don't mean to be rude but some of the things some suggest must feel I'm not too bright at this despite my being a full time RVer since 2007, near 30k miles on the road most in the rough, and never had issues with black or gray tanks.