Slide valve not closing? You might check for....

It looks like the blank from when they cut a hole in the tank. I've heard of it many times before.

B.O.

It happens. Sully (RIP) found a soda can in his black tank. Bought RV brand new.

Not sure what it is. It doesn't seem to match any materials used in our unit.

STBRetired, yes, you are right! Better this than the scenario you mentioned!

can you tell just what it is exactly?

Better that than the opposite problem of won't open. At least the tank can be rinsed out and empty before you have to pull the valve assembly.

UGH what a mess. At least you found the problem!