We have a 16' with the combined black and grey tank. Yes, it fills quickly. especially if there are 2 of you showering every day and washing lots of dishes. And yes, it will start to smell after not too long, UNLESS you pour in a bottle of this stuff, which I've found to work incredibly well:
There are ways around your tank filling too quickly while boondocking. Use the handheld outdoor shower. Or take a wet-wipe bath! Goop brand makes a good one. If you're in a campground with an outhouse, use it, especially for #2. This way, if you have to empty into a blueboy, it's just grey water. Use paper plates and cut down on washing dishes. They make good kindling for your campfire. Once you get going and you're thinking about it, it's fairly easy to cut back on water going down the drains.
I have fantasies of growing into a larger trailer with separate black and grey tanks, but until then, we make it work. It's not always a perfect situation, but we love our trailer too much to get worked up over it.