I experience some increase in water temp everytime it's over 100 F. But only with the AC on. I have manual on and off for the Flexalite on top of the thermoswitch. I have it set to ~ 210 F to come on during normal driving. With the AC on on a hot day, it blows cold first but since the fan's not on because the water temp hasn't reachted operating temp, my AC cuts out. So as soon as I run the AC, I use the manual on switch for the fan.
Typically, after some 30 min. of stop and go, with AC on, my temp starts to go up. Very slowly. At speeds over 60mph, the temp needle alternates between dead center and hotter. The moment I turn the AC off, the needle goes back to normal.
So I guess the fan works to cool the engine but not with AC over 100F, that's my experience. My waterpump's new, as is the radiator, coolant hoses and everything that's attached to it. I even have an extra engine oil cooler which reduces the oil about 10F from one side to the other.
Headgasket was replaced too, about 5 or 6 years ago. Sofar so good.
In that kind of conditions, I wouldn't want to tow a boat and run the AC. Bad combination. So that makes it a truck that can't haul it's own way. Not so tuff anymore.