Yep, that's the way our Tour is plumbed too - Very frustrating. 51 gallons isn't bad for a black water tank's capacity, especially if you re-program the macerator toilets to use less water, but our 105 gallon grey water tank usually only has about 10 to 15 gallons in it after several days of boondocking. That's a lot of wasted space.
Most RVers find that their grey water tank fills up first, but in 2011 and later Tours, it's the black water tank, because a lot of grey water goes into it too. I don't know about earlier models. I'm quite sure Winnie did that, because it was a lower cost solution.
I have not seriously looked into rerouting the kitchen sink and/or the half-bath's drains, so they drain into the grey water tank. It's probably worth looking at, but I'll bet it's not going to be a very practical solution.
I emailed Winnie last year and asked them if attaching a blade valve to the dump tube, to combine the capacities of the grey and black water tanks, would cause any problems. Many boondockers do that without any trouble. Winnie's response was that they would not recommend it, because they've never tested it themselves, and they didn't know how well solids would flow out of the grey tank when dumping. They also thought black tank odors "might" be a problem in the grey tank. ???
Our grey tank doesn't have a flushout system like our black tank does, but our first motorhome didn't even have one for the black tank - I just filled it with fresh water after dumping and flushed it out that way - worked great. Right now, if our grey tank overflowed, we'd see it in the shower drain, because that's the low point. It would be a real drag to see black water bubbling out of that drain instead of grey water, but our 115 gallon fresh water capacity isn't enough to overflow both tanks, so I don't see that happening.
I don't know how high the bottoms of the black and grey tanks are in relation to each other, which is something that will affect how well they crossfeed, so that's something to evaluate. That should be easy enough to do by filling the black tank to near full (with fresh water) then opening the black and grey valves and see how well they crossfeed.
Kev