They are called tank covers.
There are at least three varieties. One is vinyl, and it's the cheapy. Camping World would have them. You often see white plastic covers on other brands, but I've never seen one on an Airstream. Again, CW would be the source. Then there's the aluminum jobs. These came from Airstream, but a good sheet-metal person could make you a nice one.
Of course, you also could POLISH the tanks if they are aluminum. These look pretty proud sitting up front.
Then again, tanks is tanks. There's nothing wrong with leaving them sitting out there for all the world to view.