Any suggestions on how to use Google Analytics to track the number of times (and exactly when) AdSense doesn't display an ad and instead defaults to the "backup" ad?

On my site, AdSense is not displaying ads on occasion. This is happening more often than I've seen in the past, and it coincided with my switching to async AdSense js. So, I need to debug whether it's a problem on my site -- or if it's just normal AdSense behavior.

Obviously, in order to troubleshoot the issue, I need to know when and how often this is happening. I thought there might be a way to get a report in AdSense but I can't find that. If that report exists, let me know that and forget the rest of my question.

Historically, I just had "show blank space" as my backup ad. I know I can create backup-ad.html and backup-ad.gif and put those on my server and designate backup-ad.html as my backup ad.

But will backup-ad.html be in an iframe, and if so, do I therefore need to put Google Analytics in backup-ad.html file? And if I do that, won't I be messing up my page views and won't I also never know which page served up the back up ad?

I know I could also use a Google Analytics event to track the image view (thus not messing up my page views) but I think I'd still have the problem of not being able to know what page resulted in the backup ads getting served up, because it would just tell me that it was invoked from backup-ad.html.

What I need to know is that backup-ad.html got served by XYZ page so I can then go to XYZ page and see if it's not doing what it is supposed to do. I have many template pages so that's why I need some data to pinpoint any issues; it'd be a no brainer if I just had one template file.

Anybody figured this out and care to share a good way to do this?

Thanks in advance.