From VanagonWiki

[edit] Vanagon fuel tank

The fuel tank on the Vanagon varies by model: on gas-powered, two-wheel drive Vanagons, it is a [capacity] tank^[1] which sits in between the frame members just behind the spare tire, right below the battery boxes underneath the cab seats.  Diesel-powered Vanagons have a [capacity] tank^[2] in the same location.  On Syncros, it is a [capacity] tank^[3] which sits just forward of the engine and, in part, above the transmission; hence the Syncro's distinctive fuel filler location above the right rear tire.