Its an old car I am no gasser expert but I would suspect the following:
Tired injectors
bad o2 sensors
Dirty plugs/air filter
Carrying lots of crap
Driving hard

650 miles out of an S420 is pushing it but not amazing. 26.5 gallon capacity means if he was getting 25mpg he could have done it. Granted he would have been an fumes pulling into the gas station. 25mpg is very doable in an S420. Figure 60-70 on the highway with the cruise on.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200