First, no such thing as a stupid question....If you are going by the indicators inside...don't always work. Your tank could be empty but still registering that it has something in it, mine does it all the time. I look and then it shows empty. Sometimes pieces of paper get on the probes.. I try to dump some of my dishwater in my black tank on just about every outing. A lot of people put a bag of ice in there...jostle it around some while towing, helps to clean. I don't know where you live but you can always add enough RV Anti-freeze in there to help for winterization. Just my thoughts...