No aux fuel tank, but there is a vapor separator/return tank. Plastic, about half the size of a football, could be anywhere in there.
Check underneath for the vent line, its got a fitting on the end that mud daubers love to make nests in, spiders too sometimes, and when they get blocked, gas fumes come out somewhere else.
Also check the rubber lines at the filler neck and the filler cap -- the one on the 280 was dumping fuel from a bad gasket, and pending a new one I put the old oil filler cap from the 300TE (got a new one, it leaked oil) on the 280 gas tank. No more gas smell.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!