Mosfet Switch not working as expected
Your circuit is correct.  Your way of measuring the RDSON is not.

With a load of 1k my Amperemeter gives me a current of 11.8mA @12V

This is good to see that the circuit is generally working, but you don't have anywhere near the accuracy to declare the RDSON result you do.  How accurate is the supply?  How accurate is the resistor?  What is the accuracy of the ammeter?  What is the resistance of the ammeter.

Note that small errors in the first three cause large errors in the RDSON conclusion.  And the resistance of the meter is likely significantly higher than RDSON.  Even if the other numbers were accurate enough, all you'd really be doing is measuring the meter resistance.

For example, let's work backwards what some of your measurements would need to be to indicate 0 RDSON.  The supply might only be 11.8 V.  That's only 1.7% below spec.  Is your supply really that accurate?  Right.  I didn't think so.  Or the resistor might be 1.017 k&Omega; instead of 1.000 k&Omega;.  That's only 1.7 % high.  What range of current is really running thru the ammeter when it says 11.8 mA?  A combination of errors under 1% is all it takes to make the computed RDSON actually come out negative!

Your measurements don't support your conclusions.