Yep, I've had this happen with a loaded Westy on long, steep descents.  That's why I went with a big brake kit a few years ago - not for the stopping power per se, but more for the vented rotors which really help with the heat and fading.  It's possible you've warped the rotors, but unlikely if you haven't noticed a pulsing or "wobble" in the steering wheel when braking.  Much more likely is the pads get heavily glazed due to surface melting which can reduce braking power, and can lead to squealing.  IME, once such episode is unlikely to cause much problem.

Keith Hughes
'86 Westy GenV (Marvin)
 
 ------------------------------
 
 Date:    Mon, 9 Apr 2018
 16:14:09 +0000
 From:    Vincent Dow <ianvincentdow@GMAIL.COM>
 Subject: Hot brakes!
 
 Howdy y=E2=80=99all,
 I was coming down a steep, windy road
 in Malibu last night, and st the
 bottom I felt that my brakes had lost
 done if their grabbing power.
 At the stop sign at the bottom  oF
 the hill I got out to have a look and my
 brakes were smoking + very strong
 smell. Like a truck cooking it=E2=80=99s =
 brakes.
 
 My pads look good, lots of material
 left .
 Anyone ever experience this before in a
 vanagon?
 Do you think I=E2=80=99ve done any
 damage?
 Or is it normal for mountain highway
 downhill?
 Best ,
 Vinnie