Hubba,
I don't subscribe to having a combined tank. I don't have one and I would be reluctant to purchase a unit that did, unless I was a camper that always knew I would have sewer hookups.
See the thing is that you will for sure, beyond question (if you are like at least 50% of most RVrs) use far more water that will go grey than you might for toilet. For example, dishes. You wash them perhaps once a day to once every other day. Sure you could lug along pans and do them outside, but to me it defeats the purpose of having the sink more or less. Then there is the shower. What if you decide to take one? All that water will go to the grey tank and if no grey tank is there, it all...you guessed it....goes into the combined tank.
Bottom line, lets say in the worst case senario, your out boondocking and your tank gets full. Now sewage combined with grey water could backup into the shower (which is usually the lowest point in the coach). In a pinch, if you had 2 seperate tanks, capture some of the grey and unload some of it to make more room if the campground didn't allow purging of grey water. Once you add sewage to that equation, it's a whole different matter.
So I don't have a combined tank, but I hope that gives you a sense as to why a good number of folks dislike a combined tank. I am sure there are other reasons both for and against. I just happen to fit more into one of the "non" camps.
