Inverter - got one on my new Eddie Bauer, but haven't needed it yet. On my two prior Airstreams I used a Honda 1000 and battery charger to recharge my coach batteries and stuff like my computer. I think an inverter may be a "nice to have" more than a necessity, even for a boondocker.
If I wanted to I could run an electric skillet or induction burner from the 1000 - but definitely couldn't even think of using the air conditioner. Even before I put LED lights in the Safari recharging the batteries wasn't a big deal - I could boondock for 10 days (until the fresh water ran out and I wanted a shower!)
I have both solar and the inverter - but guess what my FAVORITE upgrade is so far?
The power cord. It has a little blue LED in each end. Plug it into the power post and if the 30 amp circuit isn't good, it doesn't light up. Plug it into the coach and you know you have good power into the coach. It's like a multi-meter I don't have to get out of the back of the truck to use! Last night I went home and the A/C was off. I looked at the fridge - Auto and propane lights were on, opened the curtain, NO blue light special on the cord. Went outside, flipped the breaker - it was sloppy and probably bad - got the 50 to 30 adapter cord and plugged into the 50 amp outlet, went back inside and enjoyed my A/C.
I LOVE IT. If you can't afford a new Airstream, then this cord would be a great "best upgrade under $100" for your current unit. We've had a lot of bad storms lately and the power in the campground goes out frequently. I've been sitting down sewing and the machine stops, but the lights on 12 volt
stay on. One look at the power cord tells me whether to head for my internal circuit breakers or wait for the power to come back on from the grid.
Were my budget more limited than it is and I had to choose between options - I believe full awnings would be one of the most important ones. I always seem to park with the door facing NORTH where the big awning doesn't do all that much, but just covering the windows on the south side would be a BIG help. And the small awnings are so easy to put up and down that it's never a hassle. With the big awning, if you're running to the store for an hour... you've got to check the weather report before you decide to leave it up. (And I've been blessed by neighbors who've taken it down twice when the wind unexpectedly got really gusty!).
The potential occupants of the Airstream are really the determining factor on which upgrades are NICE versus NEEDED. Dogs & children make ultra-leather far more than a nice to have. Putting up the big awning isn't all that difficult for me alone, but it is a lot faster when a neighbor walks over and holds the strap while I lock the support arms. I should have salvaged the awning helper handles from the old coach, darn!
Anyway, whatever you get - ENJOY IT and retrofit whatever you regret not getting in the first place.
Best to all, Paula