Ken,
No, the extra tank was a kit installed after purchase. Car initially bought to be a "touring" car for long distance trips (back when it was harder to find convenient diesel stations). And it does indeed continue to be a great car to take on the highway - albeit minus some of those newer gadgets that a gizguy likes. My Garmin GPS works just fine on the dash
Two disadvantages of the extra tank:
1. You lose the trunk space beneath the rear window
(although there probably is room to add another 1 or 2 tanks)
2. Extra fuel weight makes for slower acceleration.
Also, may be just that the 300SD is older than the E300, but anything you leave in the trunk acquires a distinct diesel aroma. Have never found any leaks, so must just be fumes that escape through the connections.
Tom