Re: Towed my travel trailer home today...
Short answer: Its not shifting in and out of o/d that drives the temp out but locking/unlocking.
Complete answer: Haven't determined the exact value yet (should be in service manual </rant>) but over about 50% PLA (throttle possition) it will probably unlock and go into PE (Power Enrichment - can see in O2 reading).
What happens is that unlocked, the torque converter starts slipping and that generates heat so is better in 4th/locked than OD/unlocked.
By itself, 4th vs OD should not affect the MPG that much, but if spending a lot of time in PE, that will.
Back in the day, when towing you could feel the difference between primaries and secondaries in your foot. Now you need instrumentation.
Notary Sojac Y'all
One of these is a diesel: