Welcome to the AirForums! And Welcome to the exalted ranks of Interstate owners! Glad to have you with us.
1 - A better windshield cover than the one Airstream provided. You've got two options hereó one that fits on the inside of the windshield and side windows, and one that fits on the outside. I actually use both. The one that fits on the outside I use at campgrounds. The one that fits on the inside I use in storage because with the Interstate locked up no one can mess with the inside cover. On really hot days, I use both; that windshield is like a greenhouse window in anything except full tree cover.
2 - Cargo box or cargo tray that fits the hitch receiver. As you noticed, storage space is at a premium, and it only makes sense to store all of your outdoor gear outdoors on the tray or in the box rather than have it cluttering up the interior. NOTE: If you have an extended model, with its larger rear overhang, you may want to mount the tray or box to a 2" lift receiver to elevate it slightly so it's less likely to drag the ground entering or leaving a driveway.
3 - Leveler blocks. Camping in an Interstate that's not level is uncomfortable and leads to drainage problems for the sink and shower, both of which work best when the Interstate is either level or very slightly tilted toward the driver's side due to the drain locations.. Remember, though, that if you have to elevate the rear wheels, you've got to put leveler blocks under BOTH wheels of a dual, not just one.
Other so-called essentials will vary depending on where you plan to go, how long you plan to be gone, how many people are along for the ride, and other factors. But those three will get you started.
Engineering: Finding complex solutions to simple problems you didn't even know you had.