DrewE wrote:
If you have access to the other side of the valve from the handle, maybe you can drill a smallish hole opposite the rod and push the gate from behind with a dowel or another metal rod or similar.
While I haven't heard of it being done, it seems to me that one ought to be able to pump out (most of) the contents of the black tank by putting a hose down the toilet, similar to how septic tanks are pumped out. If you were really brave and/or foolhardy, a shop vac would probably do that trick...but I wouldn't want to do so with my shop vac! Having an intermediate container to collect the sewage might be practical (maybe a 5 gallon bucket with a lid, with the hoses connected to the lid). It would not be pleasant work, of course, but beats the alternative.
. Now if you will just lend me your shop vac I will return it and you can use the contents to fertilize your garden!!!!