Hey,
That does seem low but not as bad as my mileage was at first. I was only getting 18mpg when I first got it. Now I get around 25-26mpg. I believe the 300D should do better being a lot lighter.
The manual says for performance issues to check timing.
I would check the timing and correct with a woodruff key, which is what I did or some people will recommend to replace the chain if it's stretched.
Click this link for cam timing instructions, choose section 05-215
While you have the valve cover off adjust the valves. Section 05-210
As Leo said check for fuel leaks and replace fuel filters. Pay attention to the rubber fuel return lines on the injectors. He mentioned if it rolls freely. If not you can have dragging brakes or you may need to repack the front wheel bearings. The wheel bearings are a maintenence item you should probably do this anyway.
If you continue to have performance issues you can clean the injectors.
Welcome to the wonderful world of DIY!!
Danny