Black Water gauge always says full

I'm personally new to having the indicator lights myself... i used to work at a camp ground where we did "pump outs" for the non sewer sites... people would complain about their light still showing full but their tank was empty. there were some seasonal campers that had all sorts of contraptions to clean them out that they all raved about. i'm sure they all did what they were supposed to but an old timer told me to fill the black water tank 1/3 full before you use it because most people don't flush enough water when using the toilet and fill it up more before you dump it and his lights always read empty after dumping... not sure how this works long term but we've been doing it and we haven't had any issues