16-1800rpms seems a bit low for towing that load at 65mph. Ime lugging Uses a bit more fuel than higher rpms towing. Can see it in the egts as they only go up with fuel usage.
I stuck brake would have burned itself out and youd smell it if it was hanging bad enough to knock you 3-4mpg I believe.
When I used to watch mileage on my trucks there would just sometimes be worse mileage seemingly inexplicably.
Since it sounds like everything is pretty normal w the truck, runner for a full tank or 2 unloaded and see if you get correspondingly bad mileage. If it's a real engine condition causing it, you'll get poor mileage empty too.
And get a tune for power, not mpg. Good tunes will squeeze out a little more mpgs in general, but ime you end up using the power which uses some fuel!
