I didn't read anywhere in your posts that you are making sure the black tank is at least half full before trying to dump it. You need to have enough liquid in the tank to create a swirling motion to flush all the solids out. If you were dumping weekly, it may be that you were only getting the liquid out but leaving a lot of solids behind accumulating in the bottom of the tank until it finally reached critical mass so to speak. May not have anything to do with freezing temperatures especially if it only got down to 30 degrees overnight.
