Howdy and welcome to airforums.com! As you probably already know, this is the go-to place for information on your Airstream.
By the way, Aerowood is one of the guys that really knows what's going on. You'll realize that when you've been here a bit longer. He nailed it, of course, that's your water heater. I'll guess it's the original one, since it doesn't have the foam insulation they put on the newer ones.
The "cylindrical thing with a dip stick" behind the refer (fridge) is part of the heating system that cools the interior of your refer. That part gets hot (the "dip stick") so don't touch it if the refer has been running for a while...
Your black tank is directly below your toilet, which, if it's like mine, is below the floor.
Keep asking questions, you'll figure it all out sooner or later. That's how most of us learned about them!