Table Salt in Gray and Black Tanks?????

Thanks everyone. I use calgon now, but was hoping that salt would work as it would be a lot cheaper. We had a problem last year and yes the tank read 2/3 when empty (gray) and the black reads 3/4 even when empty. I put gallons and gallons of vinegar in both black and gray on our way home last year- used ice etc. let sit when we were home and finally it cleaned everything. I am just trying to prevent the same thing from happening. We don't really need the guages to work correctly, but I like for them to be working. LOL