I've found a couple of post on different website on how to make individual voltmeters and ammeters, but I haven't found any that combine the two.
The Arduino has multiple analog inputs. 
It's actually easier to measure high current. A hall-effect current sensor is probably the way to go. If you use a shunt, it has to go on ground-side, because the Arduino can only measure voltage referenced to (its) ground. And hopefully, you know how to calculate power dissipation in the shunt.
For voltage, the [u]Analog Read Voltage Example[/u] will get you started. You'll need a voltage divider (2 resistors) and you'll have to scale the readings appropriately. And, I'd recommend a couple of [u]over voltage/reverse voltage protection diodes[/u] in case of unexpected spikes from the motor or charger.