By the look of the soot and heat damage on the trailer id say there was a cross wind blowing, thus keeping alot of the heat off the trailer its self.
Something helped out — figured if it was hot enough to burn down like that, the heat transfer would get to the trailer.
The trailer probably would have went up to if it had been a shorter wheelbase truck... There looks to be at least 4 feet from where the back of the sleeper was to the front of the trailer... Lots of air space + possible cross wind + the fires natural draft = insulated barrier of fast moving air.
I had a tanker with a load of stove oil burn up. Aluminum cab, aluminum frame rails, aluminum tank. The only thing left when it finally burned itself out were the rear end housings, the steering axle, the lug nuts and the pintle hitch. Everything else melted to slag.