When I was in the Navy, I went to both Tropical and Cold Weather medicine schools. Both said that a rectal temp was essential to get the proper core body temp in "environmental injuries"
Once when on deployment in Okinawa, the battalion colonel decided that standing orders did not apply to him and his Marines, so he took the battalion out on a 25 mile hike with full gear...in July. The Preventive Medicine department had already called the day as "Black Flag," Heat and humidity so high that absolutely NO outdoor activity was to be done unless absolutely necessary.
I was working the Battalion Aid Station as Senior Clinician (The MO was on leave in the Philippines.)when the call came in to expect heat casualties. I cranked every AC unit in the building, had some D5W and D5NS chilled, and plenty of cool oral fluisds handy when they started to arrive.
As per procedure and training, EVERYONE who came in got a rectal temp, whether they were walking or being carried in.
The Marines seemed to understand when I told them what we wwould be doing and why. (They also knoew they had no choice. Medical necessity trumps rank.) Oddly enough, it was the younger "green' corpsmen who had the biggest problem with it.
Yes, we used IVAC's with red probe tips, not glass.
To my knowledge, that i still standard SOP.