So two things to check. 1. Air can be checked cold by jacking up the front and removing the radiator plug. Massage the upper hose to release any air. Might not need to use a jack, depends on how full the coolant recovery tank is and how it levels with the radiator. There shouldn't be any air under the plug. Top it up if needed. 2. The stat can be checked on the stove top with a candy thermometer in a pan of water. Taking the stat out of the truck helps simplify this check .

99C is not awful, especially at a standstill on a hot day. It will barely budge the dash gauge. I measured 104C the other day waiting to pick up a sandwich, with my new 88C Waxstat t-stat and my two row aluminum radiator. The dash gauge was just over the second wavey line, barely. My 50/50 Prestone coolant is rated 130C for boil over protection (unpressurized). Seems you all up north better get used to hot weather, setting records and all...

 

210F isn't horrible for a D1 with AC running if it's hot outside. My D1 that normally runs around 180F, sits at 200F with the AC going in ambient temps of 85-100F

I missed whether your electric fans are both running ?