Bizarre holding tank problem! It fills up on its own!

Leaving your grey valve open can allow solids to remain (food debris/grease even body oils/skin, soap/conditioner residues etc.) which can build up just like a poop pyramid.

Hey OP
Just what is your grey tank capacity?
Your black tank?

I have a 50 gallon black......2 adults FT ----using toilet exclusively (no cg toilets) we go 10 days before dumping
Grey is 2 tanks (galley 28 and grey 50) with the 2 tied together same 10 days
Otherwise galley would need dumping on day 4