My wife and I just got home from our shakedown trip in our 2018 27BHSS. We spent 6 days at a site with full hookups and it was just the 2 of us with the occasional visitor while at the campsite. I tend to keep the valves closed until a tank needs to be emptied so the following information may be somewhat useful.
I emptied the black tank at the 2/3 level on about the 5th day. It was still indicating empty when I emptied it prior to leaving the campground.
The gray tank for the kitchen never became full. I had to empty the gray tank for the bathroom 2 times - but my wife doesn’t understand the concept of conserving water during showers. In July we will be traveling from Utah to Wisconsin with our daughter and 2 of our grandsons and will have a better feel for tank management by then.
We will also be doing some boondocking for a few days in late July, we will really get it figured out then I suppose...