Separate names with a comma.
Seems obvious but NYDutch... you no longer have a Gray tank it's now a Black tank. When you mix those two like you're doing it goes both ways.
When stopped for more than overnight we keep the Gray valve open all the time until the day before leaving. This way we don't worry about a longer...
OK, this isn't going to be a popular answer - nor delicate. Some of us prefer to not... er, defecate in our RVs if there are nice restrooms...