Airstream Interstates aren't exactly the ideal boondocking vehicles. But with portable panels, it can be done.
If you're going to be in the Big Bend area, then you should have plenty of daylight to work with.
First step, as I see it, is to figure out your 24-hour electrical usage. Leaving out your air conditioner and microwave/convection oven, of course, because they won't run off the inverter anyway. Side note, get some Prodex to cover all of your windows (even the tinted ones) and windshield during the day, because without that, you'll bake in the Big Bend area. Those big windows are like a greenhouse!
Once you have your 24-hour electrical usage, multiply that by 3. That is about how much solar capacity you'll need to keep your batteries charged on a day-to-day basis from solar alone. Multiplying by 3 rather than 2 accounts for the fact that even though you might have 12 hours or more of daylight, unless you want to be constantly re-aiming your solar panels, you'll have time when you're not using them to full efficiency.
In my opinion, your best bet for wiring in portable panels is:
1 - Do not
use the charge controller for your rooftop solar. Use a separate controller for your portable panels.
2 - Open up the battery box and make separate connections from the portable controller to the batteries. Use a two-pin plug outside of the battery box so that you can simply plug in the portable solar when you have it hooked up and can unplug when you're not.
2A - Alternately, you already have a 12v
power outlet on the right rear quarter panel of the Interstate. You might repurpose that to plug in your portable solar, if you're not planning to plug any 12v
appliances into that outlet while camping. I'm not an expert on that, though, so you'll have to work out the details for yourself if you want to try it. I don't know if you could safely back-feed 12v
power through the outlet or if you'd have to rewire the outlet to turn it into an inlet.
3 - Get a cargo box that fits the hitch receiver to carry your panels. That way, you don't need folding panels and can get bigger and/or better ones.
The statements above are my opinions alone and do not necessarily reflect the opinions of management.