I have a 2000 390 XL as well, after owning a 2000 XC (35' diesel) so they both had the same windows. Both units had the problem, one worse than the other. A couple we travel with have the 2000 gas model, and his is the worst I have ever seen.
Yes, it is a problem. No, the factory has refused any responsibility as far as I know. The question was asked at the International Q&A session at Lansing, MI two years ago. As long as the unit is out of warranty, the windows are your problem. Please don not take my response as sour grapes, I just answer bluntly.
Now, I chatted with a company at the FMCA rally in Perry, GA a year ago. They will rwplace your window with theirs, costly, but it came (at that time anyway) with a longer warranty- seems as if it was 5 years- please don't take that statement as pure fact. They were located in Elkhart, IN if I remember correctly.
Now, back to the subject. Apparently, the gas or atmosphere that was put between the two window panes expands at a different rate than the outside atmosphere, and allows the gaskets to be either pushed or pulled into the cavity.
No know treatment of which I've been made aware. Oh, and the manufacturer of the window disavows responsibility too. I imagine, if it were possible to get evry A/S motorhome with the problem signed up into a group we possibly could get them to do something, either legally or through publicity (negative). I'm afraid that might be difficult with the history of Airstreamers being divided into so many camps on any and every issue!
Good luck if you chase it. Keep us advised....and welcome to the forums. Not much late model motorhome activity here though.