I think I know the reason so many people claim the explosion occured way behind the suspected truck: the CCTV camera filming from behind the suspected truck has a quite narrow field of view, making the part of the bridge that climbs look much steeper than it actually is (lens compression). For the same reason, the preceding flat segment looks shorter than it actually is. If you then look at other footage of the aftermath, the damaged segment doesn't seem as steep, so it's easy to misidentify it in the explosion video.
Looking at satellite images, the part that climbs towards the top segment is over 1000 meters long and consists of 4 segments, with expansion joints between them. Those 4 climbing segments are preceded by flat and level segments. I labeled 5 relevant segments of the bridge visible in the explosion video over a satellite image (another two preceding segments are actually visible in the explosion video, but I didn't include them to keep things simpler):
A - 260 meters, last flat and level segment before the climb, not damaged; the surviving truck was probably on this segment at the time of the explosion;
B - 260 meters, first climb segment; starts flat then slowly pitches up; this was the segment destroyed by the blast;
C - 260 meters, second climb segment, flat, not damaged;
D - 260 meters, third climb segment, flat, not damaged;
E - 320 meters, fourth climb segment, begins to level off, not damaged;
The CCTV camera is probably at least 500 meters further back behind those 5 segments. To give you an idea of how much the field of view effect can fool you, I'll include a dash-cam video of crossing the bridge:
In this video, at 10:58 we reach the beginning of segment A, on which the surviving truck stopped. At 11:09 we see the beginning of the destroyed segment, B. At around 11:16 we reach the most likely point for the explosion, based on the scorching on the opposite lanes. At 11:20 we can see the beginning of segment C, which survived. At 11:31 segment D begins. At 11:42 the final climb segment, E, begins. That segment slowly starts to level off, and we reach its end and the beginning of the top segment at 11:56.