[CAmark]

Vogtman,
We recently bought the 2720SL and as I recall the microwave option comes with a convection oven function as well. It was pointed out to use by our dealer that as long as you have electricity this is fine....but if no power, no oven. We chose to go with the standard gas oven and carry along a small microwave (about $50 at WalMart) for "electrified" cooking.
As for the cabinet, we found a similar one, though plastic, at Target for $30. This sits nicely beside the fridge when camping and slides between the sink/oven when traveling.
The other feature we opted out of at purchase time was the chair. With just two of us we did not want to use the floor space that way. We bring in a folding chair or two when we need the extra seating.
To expand on Leon's post -- the blackwater is stored in the toilet. When time to dump you hook up the hose outside, pull the valve on the toilet, then pull the valve on the outside. Then as Leon says, you pull the valve for the gray water dump, this flushes the blackwater out of the hose.
Good luck with your purchase, Mark