yep... even though it did not have any active codes i have seen sometimes certain codes like the stage II overheat put the truck in limp mode or even shut it down and wouldn't let the truck return to normal operation till it was reset, at least on the mcfa trucks. Not sure about the nissan ones, i'm sure they are very similar.
Missing reservoir bottle?
yeah it sounds like some shade tree work to me too...
maybe the operator saw it leaking or cracked and just took it off... who knows ;o)

Anyways about that radiator and hose stuck in the hood insulation... check inside the radiator real close and make sure its not gunked up with that red brown looking gunk that builds up from mixing different types of coolant... also look inside that reservoir hose for the same condition. When you change out that temp sensor if its located at the bypass inlet on the backside of the water-pump on the alternator side, check that bypass hose fitting inside and make sure its clear and not gunked up.
Those are some trouble areas i've ran into in the past where clogging occurs from muddy jelly coolant buildup from mixing wrong coolant types together. And of course make sure the flu's are clear inside and out and you have good circulation in the radiator.

good luck