Wow. I've always tied aftermarket guages to the nearest hot source and always had a good reading. If I dont find an issue with grounds or wiring, I'm going to wire the guage to an underhood source.
You can hook the voltmeter anywhere you want to, depending on your needs and what you want to monitor. The factory voltmeter is tied to the hot wire that feeds the instrument cluster. If you want to hook one to the battery by all means do it, but a connection inside will give you useful readings, just not the same as at the battery.
There is a large yellow wire that leaves the starter relay on the fender, comes inside the cab and goes to a splice near the ignition switch. it splits at this splice and feeds the ignition switch and feeds the hot all the time part of the fuse box. I would take your voltmeter and take a reading in the fuse box. If it's low and you want to test the poor ground theory(which is a good one) take a scrap piece of wire and hook it to the battery negative, run it in the cab and hook it to the black meter lead, and then take your readings again. If they are higher then it's definitely the ground. If they are the same, I would start out at the starter relay and start uncovering the harness and the yellow wire. There are splices in this wire out there also, and they are known to corroded sometimes.
