First i looked on the Internet if there were more people that had this idea. Sure enough, after some searching I found several videos and pictures of working RC excavators that did not cost thousands of dollars.
They were converted excavator models from "Bruder". Yes, childrens toys, but I must say that these models are very detailed. So i bought one and got to work.
Step 1: How it works
Step 2: Servo placement and controle
So you have to replace the servo controllers for a CTI controllers.
You will need six hacked servo's in totall. Three for the boom, one for swinging and two for driving the tracks. In the pictures you can see how i placed the servo's in the boom.
You have to reinforce the boom with aluminium plates in serveral places, otherwise it will twist when you are digging.