Thanks all, for the replies. Interesting reading.
The last time we acquired a new truck/trailer combo (ten years ago, and recently ordered a new truck), I logged each gas stop along our usual route (because of a grand-daughter who demands my attendance!!! at least until she gets into her teen years, we now only vacation/drive to/from Vancouver, B.C. and Denver, Co.), just to give me an idea of the fuel cost/miles per tank for the, then, new truck. Never kept those records and never recorded after that.
When the new truck (current is a GMC 1500 6.2 gas, new will be GMC 2500 6.6 gas) arrives, I will likely log gas stops on the first trip, again just to give me an idea of fuel costs/miles per tank for such a trip.
As my new purchase will be locked in for at least 5 years, mpg becomes moot to me, as it will be what it will be.
Having said all that, I understand the various reasons given by others, and certainly don't knock anyone for those reasons.
Thanks again for all the replies,
Marv