Brakes:
Sounds like you suffered brake fade and may have boiled the brake fluid as well. I've seen car/4wd with nothing on the back suffer similar coming down the gap.
Riding the brakes all the time, even lightly, prevents them cooling down so you need to use gears when coming down long/steep hills (that is why you always see "Trucks/Buses use low gear" signs) and moderate you use of the middle pedal.
How heavy is the van
Has it got functioning brakes
What gear where you in
How fast were you going
How much where you using the brakes
Overheating:
Where you towing when the KJ overheated?
Being only 36 degrees and putting the AC on sounds like there is something wrong with the cooling system in your KJ. I'd suggest changing the coolant (and a good clean at the same time), make sure the fan clutch is working properly (IMHO change it, don't faf around) and make sure there is nothing blocking airflow through the rad.
I drove through the heat wave in SA (49 degrees showing on the outside temp readout thing) a couple of months ago with the AC blasting as well as it could and my KJ never went over half, the elec fan was running as hard as it could and never switched off though