[quote="Token"]Travel trailers do the same thing but at a slightly lower frequency. My 26' trailer resonates 40Hz bass.
You are correct about the resonator concept. Standing wave will fit the length if the vehicle and amplify. Typically 1/4 of the wavelength will be the magic frequency.
Damping the vehicle will not do much. Just give you cleaner bass.
Things you can do:
If the source of the sound is nearby, park orthogonal to the point of sound origin. Have the smallest dimension of your van face the sound source.
Open the back door if you have one. This creates an infinite baffle and reduces resonance, but now you get to hear the playa. Maybe hang a blanket in the opening.
Reduce the internal length of your van so that it does not pick up the base. A well placed foam board like they use for hexayurts or somesuch.
And finally, make it a convertible.
Thanks much for this informative response.