Lippert doesn't make frames for Airstream. I'm not too worried about Lippert frames anyways. They had some issues several years ago but haven't heard much trouble in quite some time. They went thru a lengthy training and certification for all of their welders which was certainly a good thing.
I do believe the Lippert frame issues were somewhat exaggerated. Lippert has more frames on the road than any other manufacture by a wide margin. So they will have a higher # of problems showing up. I haven't had any frame issues myself but did have some minor brake/axle issues. Lippert was 1st rate in resolving those issues.