actually, my system looks like that:
the alt is prety old. even with a new voltage regulator, it still has voltage drop (not less than 13v) when it has big load on it..
isolator is a diode type. gonna change it with relay anyway
bat1: is connected to chasis, via 2 fat wires (that bot to the chasis)
carpc: use the M4atx dc-dc
the lcd is powered from the m4-atx, and has VGA signal. (15.6" )
the signal from the carpc, to the amplifier, has to pass through an RCA ground loop isolator, or else it get much noise in the speakers (i still get noise, during pc startup, even with the isolator)
the LCD, has noise on the image.
both noises increase in freq, with the engine RPM...
[Edit]: Least but not last. the LCD is a 15.6" home monitor, that i have hooked u.
at the begining, i had a smaller monitor, with the M4-atx. no noise then
then, i tried to install the big LCD, wich was powered by an inverter (Since ir required 220hz). that is when the noise firstly came.
then i used this: i had an inverter, that powered both my pc (DC->AC->DC from home PSU) and the Monitor no noise that way.
after that, i changed the LCD's power supply, with a DC converter, so now it took 12v, and 5v from the carpc PSU. still had noise with the M4-atx, but now i also have nosie with the dc-ac-dc build...
also, in all those cases, the longer the VGA cable, the stronger the noise( no change in freq ofc).. so, i HAVE to consider, that maybe the LCD intself is the culprit...