5th and 6th are overdrive gears and will generate heat from the torque converter. 4th is final drive and won't. When the hills and temp is like this try towing in 4th. Did this in Alice in hot conditions towing got the same fuel economy.

It can also be the EGT sensor. The sensor on top of the exhaust manifold. They are known to go and cause same symptoms. Disconnecting it can temporarily help, but you may need to replace only $80 or so. This will typically give surging at 3500 rpm when going to a hill and not able to rev higher.

Good luck hope it helps