What is the typical temperature of an airliner's hull during flight?
There are two primary factors that affect the skin temperature of an aircraft in flight: the air temperature, and the speed of the aircraft.

The air temperature where airliners cruise is relatively cold, around -54 °C at 35,000 feet.

As a body like an aircraft moves through air, it compresses the air, which causes the air temperature to rise. The maximum temperature rise will be if the air is completely stopped, such as at a leading edge. This is called the total air temperature, and the amount that the temperature rises is called the ram rise.

Using a simple formula to find the ram rise:

$$RR = \frac{V^2}{87^2}$$

… where $RR$ is in Kelvin, and $V$ is the true airspeed in knots.

Using a typical airliner cruising speed of 500 knots gives a temperature of 33 degrees. This brings the total air temperature to -22 °C, which is still quite cold. At places other than the leading edge, the temperature rise will be less. This is why cargo holds will need heaters to be safe for live animals, even being insulated and pressurized. Airliners just don't fly fast enough to produce a significant amount of heating.

On the other hand, the SR-71 could fly at over 1,910 kts, which gives a ram rise of 482 °C. The air doesn't get much colder as you climb to the altitudes where the SR-71 flew, so this gives a total air temperature of over 400 °C. Speed makes a huge difference.