Based on what I could see from several videos, the source was the RV. From the characteristics of the explosion, the device appears to be a chemical percussion device in nature. A fracture appears have occurred as the pressure initially built up, causing an out gas stream at the back of the RV upwards just before the primary blast. As the pressure of the blast hit the nearest building, it forced most the energy of the blast back and across the street to the other buildings so it looks like the explosion originated there.

Just my opinion of course.