Path of least resistance. IF you build a bomb in the passenger compartment of an RV you’re at least a few feet off the asphalt. The fiberglass or light gauge metal and glass in the rv offers very little resistance whereas the downward blast wave is going through the frame of the vehicle and more substantial components. Think of the little firework on a palm versus one held tight in a fist. Way more damage to your hand if you contain the blast.

Granted, I have not gotten into an analysis of photos (Christmas with family).

Blast waves can reflect and don’t always behave in the absolutely most obvious ways, but things detonated above ground rarely will leave much of a crater. Look at photos of Nagasaki after the blast.