joined:Apr 1, 2016
No, the site is not responsive.
Do you have a mobile site?
If no, then forget about everything else and get to work. Google is shifting to the Mobile first index, meaning that the content of your site will indexed based on the mobile version of your website. Showing mobile users a desktop site is not a good experience and if Google is judging your content on that basis it is no helping.
Furthermore, my guess is that many advertisers are not keen on showing desktop ads to mobile users, so you may have very little demand for your mobile impressions. Low demand == low price.
Also, if you do have a mobile site, it is the content of the mobile site that will be used in the search and ranking index. If you have removed content, and features you are loosing the value of those in your rankings. All this will lead to fewer users over time.
Now regarding your jan 2017 vs 2018 figures.
2017 RPM = $1.47
2018 RPM = $0.74
So a change in traffic resulted in drop in revenue of about $1.68 for that 1 day and the relative loss in traffic or PVs is only 2.6%. I don't know how much traffic variation have from day to day, but that may not even be significant. Clearly the issue is a drop in RPM.
So the next question is has this been a gradual drop over time. To check this go to performance reports pick the date range from Feb 1, 2017 to Jan 31, 2018. Compare that to Feb 1, 2016 to Jan 31, 2017. Select monthly figures and select Page RPM or Impression RPM. This should show you a graph over the time period and you should be able to see a trend if there is one. Don't get too fixated on the specific monthly figures because there is seasonality, focus on the difference between the two lines. Do they diverge? When does it start?
But again, in my opinion "Mobile First" is your issue. You can confirm this by repeating the same report as above but for Desktop and Mobile and seeing if you see significant difference between the two.
Here is some more discussion on the mobile first topic
Yes i have been using the ad-balancer throughout