Coolant loss DD15, goes from max fill line to minimum in a month.

Maybe it don't like to be full run it awhile after it drops and see if it continues to drop

 

One place that I have seen leak an most people don't see it is under the sleeper heater system. Crawl under behind the driver seat area. Couple of lines maybe leaking

 

No. 1 gallon a month isn't too bad. Obviously you want to fix it.
Check all radiator hoses.

 

I just fixed my 2016 dd15. Hard to find leak, just weeping and evaporating. Was a hard plastic line on left side of motor from the t stat area down to the air compressor. Checked line with paper towel. I changed both lines off of the compressor, all good. Check you tube for videos.

 

Take your roll of toilet paper and put a couple wraps around every clamp/water fitting. Bring it up to temp and you'll find that tiny leak.