You can put the voltmeter wherever you have convenient access to the 12V supply and the 0V return. If you are taking this from a switch be sure to connect the +12V wire to the side of the switch with permanent 12V on it and not the other (switched) terminal. Connecting the meter directly across the battery will give you the most accurate reading as the meter will not be subject to any voltage drops in the van wiring.
An inline fuse is a very good thing incorporate wherever you decide to install it.

Cheers,
Steve.