[tangerine911S]

For something as large and slab-sided as a Vanagon, sound deadening is a must to mitigate panel resonance which keeps outside noise out and inside noise in.

With the huge amount of space behind the driver, for hifi sound you would not want any rear speakers at all. The time delay of the rear speakers would affect the sound quality and imaging. Since music is normally recorded in stereo anyway, rear speakers are many times left out even in smaller cars when SQL (sound quality) is the goal. Some keep them for personal preference, if you often have rear passengers, if watching movies with 5.1 sound, or if SPL (sound pressure) is the goal. For a 2.1 setup, these speakers should have a high RMS wattage rating. Brands to consider are Focal, Dynaudio, DLS, Rainbow...

A subwoofer would actually do quite well in a Vanagon. The limiting factor in making a sub sound great in automobiles is most often space. The amount of space available in a camper would allow for a really well designed sub box (transmission line, band pass, large ported..). Also, the shape of the camper could allow for good SP levels as the shape could let the sub be corner loaded.

There are many choices for head units but get one with a good Ipod interface and a higher pre-out voltage for the best quality sound.

Amps should ideally be separate for stereo channels and sub channel. One with low floor noise and plenty of head room for the speakers of choice should be selected to avoid signal clipping at high volumes.

Additional equipment to consider would include a parametric equalizer and omnidirectional microphone and program to tune to a flat response or to taste.