Was that in OD?
Temp's seem high to me, any headwind or a slight grade and you will be too hot. 1200ish is about the max you want to run for extended periods.
My SD in stock form (injectors) would normally run around 700-900 deg. loaded with about the same weight and with a slide-in camper in the back in OD. Lock out the OD and temps would drop to sub 700 deg. range.
Have you checked for boost/ exhaust leaks?
Might be a good candidate for an IC or some water.
