I have an idea!
Recently, something like that happened to me as I was filling the fresh water tank.
I crawled under and found that the probe that senses 1/3 full had popped out.
( there are actually 3 probes in the tank) and probably 3 each in the grey, and also black tank. So, crawl under and see what you find.
I couldn't put it in by myself, so being under warranty, I drove over to Buffalo RV who repaired it for me immediately, and checked the others for integrity.