Well, I suppose that you were running the A/C while idling for an hour. Tunes?
Most 88 300E cars in such a condition and new original alternators will kill the battery about as fast as leaving all the lights on including entry lighting. The alternators on 88 300Es make about 20A at idle. The fuel and ignition use a good portion of that probably 15A. The A/C with aux fan will easily take 30-40A. Add it up. A set of headlights probably use 6-8A.
You are using more than you are putting back and thats all you get in 88. A big improvement can be made by buying the 1990- harness and 80A alternator. The fact that the overall output is 10A higher isn't the point. Improved alternator design has the 80A alternator making most of its output at idle speed, probably 50-60A.
Bosch Master, ASE Master, L1
33 years MB technician