Looking for 'Real facts' answer to mileage question.

TT and TV as per sig.
TT about 8000 lb. TV is work truck - no fuel computer
Level ground, no head/tail wind.
100 km / 60 mph

Scenario 1:

Tow/Haul off, cruise off running 5th gear approx 2250 RPM, transmission almost ready to down shift. Manifold vac probably low.

Scenario 2:

Tow/Haul on TV will drop into 4th gear 3000 RPM. Engine not working as hard, manifold vac probably fairly high.

Weight and speed haven't changed. Has mileage changed?

Stu