The first time we camped in a thunderstorm, the front bed got a bit wet. When the service center couldn't find any obvious leaks around the front windows, they pressurized my trailer and sprayed the exterior with soapy water. The bubbles showed the location of water intrusion.
In my experience, the cost of this diagnosis and the subsequent repair was covered under my manufacturer's warranty. I understand that the pressurization process can be a bit expensive, so Airstream doesn't like to cover this as an initial diagnostic. However, if your dealer has sealed everything and you're still getting wet inside, it might be time to ask for it.
If for some reason your service center doesn't have this capability, take it to someone who does. Be sure to get pre-approval from Airstream for any non-Airstream shop repairs.
2021 Classic 33
2020 GMC 3500HD Duramax AT4