My husband and I just picked up our brand new fifth wheel two weeks ago. We are living in it full-time at a place with hook-ups so the grey water tank emptying isn't really an issue for us. However, with the black water tank we were told not to empty it unless it's at least 3/4 full. There were a lot of reasons but the one that sticks in my mind is that you want the chemicals you've put in to do their job and break down all the solid waste so that you don't get a clog somewhere when you're dumping it. We kept an eye on our gauges and did our first dump yesterday (the indicator said we were full) after two weeks and five days. Hope this helps!!