Read Voltage at PWM off time, and Current at PWM on time
Add a pin-change-interrupt (PCI) handler (See eg PinChangeInt at arduino.cc); clear the flag and enable the interrupt whenever you want to take readings, and disable that PCI after you have the reading.  As noted in the link, PCIs are available on all signal pins (possibly not on A6 and A7 on the 328), so are available on any of the PWM pins.