If there are no visible signs that it is leaking externally, I'dguess the crankcase seal is bad.

 

Update...some fluid in beer bottle. I don't think enough to equate out to one gallon per month.
Will run for the next few days to see if amount over time adds up.

Still clean in the oil and transmission. Will pull heater core over weekend to see what I can find.

We are clearing out a bay in the shop next week and doing some major diagnostics on truck to include new pressure test, new combustion gas check, checking all engine codes etc. to see if we can find something. Also replacing axle joints and seals. Over 122,000 miles on truck time for some TLC.

Thanks all for input...will let you know what we find.