I know how to search in Google Drive, and how to view old revisions of documents. But is it possible to search all revisions at once?
There is no way to search for strings from old revisions of your Google Drive Documents, even if these old revisions are still accessible.
Search results concern only the current version of your files and not the older versions.
Except that be aware that Google Drive may delete your older revisions after 30 days if you exceed the 100 versions per document unless you mark the file for "Keep file versions forever" which allows up to the latest 200 revisions to be kept forever.