You guys with multi amp set ups. Where is your earth ground at?

Mine is on the inner spare tire well. Cleaned with a air grinder with steel brush. 4AWG to a ground Distribution block less then 2' away.

Right now for some reason im losing voltage to my amps.
Alt i am seeing 14.3-14.4 Volts
Batt im seeing 14.2-14.3
ANL Fuse holder W/display 13.9-14.1 even on output with DMM
Cap input 13.9-14.00
Power block 12.5
Power at amps 10.9-11.3

I know this has to do with grounding. Any help? Any better grounding points in trunk?