With regards to above mentioned location to connect a voltmeter hopefully you will install some sort of in line fuse to protect the voltmeter. without a fuse the full current from the battery will be avaible to "nuke" the voltmeter if it faulted not good too much smoke , my prefernce would be to connect it to the ignition system eg temp gauge positve terminal fuel guage positve terminal etc check with a test light to find the positive terminal with the ignition on the earth side of gauge to any clean metal nearby
