I think it is pretty simple - most people do not use enough water when flushing the toilet in an RV. Adding water from sink (it is pretty small) along with soap residue, keeps the black tank materials liquid. Anyway, the black tank is bigger. We always fill up our gray tank well ahead of the black tank.
Roger & Mary
2017 Winnebago Navion 24V
2014 Tiffin Phaeton 36GH (Sold)
In our Minnie Winnie the bathroom vanity is directly over the black water tank. I'm thinking simple economics and ease of motorhome assembly here. Since bathroom sinks are not used much, I see no drawbacks to draining it into the black tank.
2016 Minnie Winnie 27Q on a 2015 Ford E450 chassis. Retired U.S. Air Force. Living in Anchorage, Alaska
Actually this is beneficial especially when boondocking. If your gray water tank fills up faster than the black water tank you can use the bathroom sink instead of the kitchen for some or more of your cleaning needs which can extend your stay before dumping. My Itasca Spirit (Mini Winnie) is set up this way too. From a plumbing perspective, the black water tank is much closer to the bathroom sink and easier to connect.