Actually the I think the expected direction of change would not be filling the gasoline tank, but instead if you drove and used up some of the gasoline in the Highlander between the first and second readings, lowering the Highlander's weight by however many gallons times ~6 lbs per gallon.
So as an example, if you drove around and used 10 gallons of gasoline, your equation would change from what you originally wrote:
"new tongue weight is 5360 - 4940 = 420 lbs."
to an adjusted highlander weight of 4940-60 lbs. Like this:
new tongue weight is 5360 - 4880 = 480 lbs.
It sounds like with possible WDH interactions you got things figured out one way or another! It's always reassuring to not be defying the laws of physics
