I have found that my 1980 300SD holds 25 gallons of fuel and once I got 750 miles from a full tank(30 MPG). This is more than what the manual says. I know it will take this much because I have able to put in 25 gallons when empty. There is about 2 gallons when the reserve light comes on. I have always been able to go 50 miles after light came on solid.
Cruise control: Up is to increase speed. When you let it return to center, speed should stay there. Down decreases speed and again speed should stay there when returned to center. Pulling toward you returns to last set speed. You would use this to resume speed after using the brake. Pushing away turns off, same as hitting brake, and last set speed is still in memory.
If it doesn't work like this or jerks around you probably need a new or rebuilt control circuit card or vacuum cylinder repairs.
P E H