I see two problems here -

First, you can't measure voltages that have a reference more than about .5 V above or below GND. To solve that problem you can either:
A. Build a one or two op-amp level shifter or
B. If the 20 V is stable, then just measure the drain voltage with a GND referenced PIC
and the voltage across the load will be (20V - Vdrain).

The second problem is that you appear to be using PWM to drive the gate. The voltage
across the load will be something like a square wave with a near 0 to 20V swing.

Your A/D will need some heavy filtering to give a meaningful result. And of course, the input of the A/D will need some sort of voltage divider to make sure that it doesn't see a voltage higher than Vref.