You do NOT want your sinks connected to a black tank. If you are going with a permanently mounted toilet and black tank, you will need a gray tank for sinks/shower. Toilets have a valve that essentially seals out odors from the black tank. Sinks do not have such a valve. Granted, the trap under a sink generally has water in it and that would seal out odors, but road travel (especially off-road) could result in your black tank being directly open to the interior of your trailer. No bueno. I've seen some trailers where the sink just drains out the side of the trailer and you basically hook up a hose and run it away from the trailer. I suppose this could work out if you use bio-friendly soaps and make sure you aren't rinsing a lot of food bits down the drain.
-croix