my 76 w200 power wagon reg cab, long box had the big plastic tank on the drivers side, the auxiliary tank was metal and behind the seat, the electric switch/valve was below the right rear corner of the cab IIRC. There was no extra tank on the right side, but I did have an optional tool storage box on the right side of box behind the cab.
FYI, my metal tank rusted out because the rear slider window leaked slightly.