I am not familiar with some of the details of your MB but there are a couple of things I have done in the past which have been very effective.
If the cooling system has an engine driven fan whether thermostatic or strictly mechanical, try removing it. You can get away with this in cooler weather and if you never spend too much time stuck in traffic. If it works, look into an electric fan or a manual override for the electric fan if your 300 has one (our 88 300TE has an auxilary electric pusher fan). I had amazing luck with a Fiat 128 fan on a couple of Volvos we had (3 to 5 mpg improvement).
Also, learn how to adjust the fuel injection system for overall mixture. The basic Bosch mechanical systems use a 3mm allen key (a long one) and the adjustment is betweed the big dash-pot and the fuel distributor. Only go in very small increments, approximately 1/8th of a turn at a time and keep a record of where you are at relative to where you started. Drive it for a couple of days and see if there is a difference. It is much easier to feel the difference than you would expect. CCW is leaner, CW is richer.
Have you changed the O2 sensor. You can get a Bosch universal for about $50.00 or so. These come with a short set of leads you splice into the original cable. Waaayyy cheaper than buying the OEM sensor and factory leads.