Low-pass filter step response graph doesn't make sense
Clearly the output of the filter is being loaded by something, and that something is not just a simple resistor to ground.  From the information given, we can calculate the Thevenin equivalent of what is connected to the output of the filter.

By looking only at the settled DC levels, we can ignore the capacitor.  This means we have two cases where both the input and output are known.  At 0 V in, you get 1 V out.  At 5 V in you get 2.5 V out.

Since a change of 5 V on the input results in 1.5 on the output, the gain is 0.3.  Since the gain comes only from the resistance of the Thevenin source, we can think of this as a voltage divider.  This divider has a top resistor of 300 k&Omega; and a gain of .3.  Working the voltage divider formula backward, this means the bottom resistor is 130 k&Omega;.

Now we know the resistance of the Thevenin source is 130 k&Omega;:



We can use any known input and output to find the voltage.  I'll use the case of 0 V in producing 1 V out.  Again working thru the voltage divider math, we get that V1 must be 1.43 V.

So the Thevenin equivalent connected to the output of the filter is 130 k&Omega; and 1.43 V.  However, it is unlikely you actually have a voltage source and resistor connected to the output.  It is more likely that there are two resistors, one to 5 V and the other to ground.

In other words, we'll assume this is what you really have:



From the Thevenin equivalent, we know that the R2/R3 voltage divider produces 1.43 V when unloaded, and the R2//R3 = 130 k&Omega;.  After solving the simultaneous equations, we find that R2 = 455 k&Omega; and R3 = 182 k&Omega;.