Aqua', A solar panel will help keep the batteries charged so you don't have to use a generator as much. Obviously, a generator too. It may be cloudy, you may be parked under trees, the days are short in the fall (less sun), and it may be snowing, so a solar panel will probably not be enough, but it helps a lot.
LED lights will use a lot less battery power. I don't know if Airstream has made them standard, but you can get better prices on the internet if you want them now. Superbright.com has good quality.
Lots of tools. You need these anyway because things break.
Read and read and read some more—there's a lot to learn. A lot can be learned here. Woodall's has a book on repairing RV stuff that's a good reference work.
You'll find the grey tank fills up too fast when boondocking. Get a dishpan that fits in the kitchen sink and wash dishes in it, dump water in toilet. Black tank takes much longer to fill up. You may want to bring an extra 5 gallons of fresh water—plenty of containers available for that.
Extra fuses in case something blows. That includes the power jack fuse. That fuse generally is only available in RV stores as it's an unusual one.
Some 2x10 board. If you get a flat, you can drive the other wheel up on a couple of pieces of 2x so you don't have to use a jack. It's much easier. And it doesn't come with a jack, so it's good to have one too. Always good to have short pieces of sc rap wood because sometimes you need it under the power jack or stabilizer jacks.
A lock for the tongue so the trailer is there when you come back.
Wheel chocks so it doesn't roll away.
Legos—they aren't called that, but everyone calls them that. They go under the wheels to level the trailer. Those 2x10 boards can help too. All RV stores carry them.
A water pressure reducer—there's one built in the trailer, but put it on the faucet so high pressure doesn't blow out the hose. Sometimes, pressure at a campground is so low that two pressure reducers reduce it too much, but that's pretty rare.
Two fresh water hoses (always white)—sometimes one isn't long enough.
Lots of sewer hose—sometimes 15' or 20' isn't long enough. Don't buy the cheapest, you'll just get a better one later.
A level. There's one that mounts on the front of the trailer that makes leveling much easier.
Torque wrench for checking wheel lugnuts.
Tire Pressure Monitoring System for trailer tires—you can't tell when you have a flat and it can tear up all that pretty aluminum when the tires comes apart. You also can't tell when pressure is too low and low pressure leads to tire failure.
Money to pay for this and many more things.