Ground question help needed

I ran one ground to the rear bumper and another to the motor.The more grounds the better!

Voltage slowly drops while idling? That's pretty common. What does it do at 'test' rpm? What did it do before relocating the battery?

Even though the chassis makes a decent conductor I've always figured there should also be a dedicated ground back to the engine and front of the vehicle if only to make sure both ends are at the same potential.

I'd run a 0 gauge right to the floor of hatch/trunk.Then another 0 gauge from motor to firewall. 2 feet of cable should be perfect.

remove paint on all grounding spots.

ford alternators will charge at a higher rate when the ambient temperature is colder, this helps to charge to battery a little faster, it will drop down to around14.1 volts when at a warmer temperature.
if you look at the wiring diagram for the volt meter, it is very simple, although simplicity aids in reliability, over time the gauge acuracy may decline.
try measuring the voltage at the starter relay terminal,(since your battery is in the trunk) and a good ground, and look at the gauge reading for a reference.
make sure if you are using a lead/acid battery, you have good ventilation, you may try a lincoln LS battery,they have a vent tube system or use a gel-cell.
the voltmeter should not fluctuate when running the car if the battery is good, as one of the batteries functions is to stabilise the voltage.
if you are still concened with the charging system, try doing a voltage drop on the cables, max voltage drop should be no more than .5 volts, although normal is about .2 volts, i am uncertain about in-trunk batteries.
good luck, jb