Coolant temperature

Engine temperature gauge on dash was lower than normal after thirty minutes of high way driving, pulling trailer. Outside temp was fifty six degrees F. Gauge needle is always left of the half-way point on scale.

Turned heat on. Had plenty. But gauge needle moved slightly lower. Very unusual. No noticeable abnormal performance affects. Runs fine. Mpg seems "normal."

Upper radiator hose and heater hoses were hot. Coolant (radiator and overflow tank) are full. No leaks.

After trip, I checked temperatures of hoses with infra red thermometer; upper radiator hose: 145 degrees F, heater hose from block to firewall was 150 degrees F, hose from heater core was 90 degrees F.

Coolant was flushed and refilled twelve months ago. Thermostat replaced over twenty years ago. Radiator cap replaced about five years ago.

Regular, (sometimes OCD) maintenance since new.

Where should I start? Thanks!