My Fantastic is the 6000RBTA, which is like the 5000RBT but has a rain sensor that closes the dome in the event of rain. So you know, the sensor works but is not perfect.
The dome on my fan is smoked transparent plastic. I've heard there is a grey dome available for Airstreams but I wanted the skylight effect in my living room.
No part of the fan hangs down below the level of the ceiling, or if it does it is no lower than the original light/vent that was there.
When ordering the fan, ask for the garnish (what they call the interior trim piece) for a curved ceiling. Also specify the color you want, white or off-white. This garnish has raised curved ends to somewhat match the ceiling. I actually trimmed and sanded mine for a tighter fit. Fantastic will send you this special garnish at no extra charge. You might also get that grey dome for free if you ask.
My commentary on whether to install a Fantastic or keep things original is this: Considering that the Fantastic Vent is standard equipment on newer Airstreams and is such a great feature to have, I think there is no harm installing one.