Google subscription and refresh token issue

[pierroo]

I am struggling to properly setup google verification of subscription.
I followed the docs to get the mandatory keys for iap.config:

googleAccToken: process.env.GOOGLE_ACC_TOKEN, // optional, for Google Play subscriptions
googleRefToken: process.env.GOOGLE_REF_TOKEN, // optional, for Google Play subscritions
googleClientID: process.env.GOOGLE_CLIENT_ID, // optional, for Google Play subscriptions
googleClientSecret: process.env.GOOGLE_CLIENT_SECRET, // optional, for Google Play subscriptions

I placed those access token and refresh token in .env file, and I got them from https://developers.google.com/oauthplayground as mentioned in the docs.

HOWEVER, the refresh token clearly expires every 3600 seconds.

I added it to my server (on heroku), it worked great for the first few days, and then a few hours ago all payments validation would crash with the following:

{"error":{},"status":1,"message":"{\"statusCode\":400,\"body\":{\"error\":\"invalid_grant\",\"error_description\":\"Token has been expired or revoked.\"}, ....

Whether consumable or subscription, the iap.validate will now always crash.
Of course when I manually regenerate the access and refresh token following the docs it will work again, but for how long??

Maybe I misunderstood, but instead of those expiring access and refresh token, is there any way to use a google service account instead?
I am not sure how this would work, the documentation may be outdated or vague about it.

Any help would be GREATLY appreciated!