Under the passenger side bed are where the battery cables come through the floor into the fuse box. You can run your monitor cables from the fuse box to the side entry way. Or, even easier, just attach the meter to the center connector of the battery disconnect switch.
But to be honest, the inverter panel behind the driver's seat already has a voltage display on it. Just turn the inverter on, read the voltage, turn the inverter off.
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Now, if you are looking at a meter that shows current draw, then you will need to connect the shunt to the battery itself (or the latching relay output cable) and run the meter wires alongside the battery disconnect switch wiring. You may have to enlarge slightly, an existing hole and reseal it, but drill a 2nd hole.