If the engine isn't getting hot, and your temp gauge dosent rise above 165, then my guess is the thermostat IS open.
An engine is a series of explosions.. hold a lighter to a spoon (even in sub 0 temperatures) and it isn't just going to stop getting hot at 165.. it's gunna keep on getting hot till something comes along and cools it.
For instance, drain all the coolant out of your truck and let it idle for an hour.. i'll bet it gets hotter than 165.. even if it's 20 below outside with no hood on it, in the wind.
