Can you tap into the trolling motor plug in parallel to add a volt meter?  Looking at the backside of the plug there doesn't seem like a whole lot of room unless you actually tapped right off the positive and negative wires but I'm not sure if that's smart.

 

For reference:

 

In that instance here's what I'd do

 

Strip off some insulation a couple inches back from the plug full circumference of the wires.

Wind a good length of the stripped end of the wire to the meter around the now bare wires.

Touch of solder to secure and protect the electrical connection.

Liquid Tape to create new insulation around the connection.

Cant you attach at the battery?

You can, but then I'd have to run wires from the front deck all of the way to the battery.  I'm currently having a really tough time finding the screws to get the front deck off for some other projects, so I'm not sure running wires is feasible.