Just took my 3500 out on it's first trip with the holiday trailer. Temp stayed mostly around 90C, but slowly worked it's way up to 106C now and again.
It only climbed that high when I went up short hills and cooled down going down the hills.

It also seemed odd that the fan didn't come on. Just hoping I didn't do any damage!

Is this something to be concerned about? It's still a long way from the red, but seems plenty hot to me.

that's hot, it converts to 223F and you don't want the 6.5 to go over 210F/99C
90C is 194F and that is right for a 195F stat.