Gauges reading wrong

Something strange is happening causing virtually all of my gauges to read wrong.
I have a Sea Ray 200 CC 1989, and there is something strange in the mix.

The voltage pins to the right + 18v, and I have confirmed this is not accurate when running with a volt meter.
The Trim gauge reads all the way to the left. Proper trim is the first line on the left when on a plane.
The RPM gauge intermittently reads incorrectly, and shows a higher RPM than what I am actually running.
The temp gauge is reading high as well. I have confirmed temp with an IR thermometer, and tried replacing the temp gauge. The new temp gauge also shows a high reading.
The speedometer also reads wrong, but I dont expect much from that one.

I would really like correct temp and voltage.

What gives?!!

Might it be a grounding issue?

Yes - most likely a bad ground. You need to go on a rabbit hunt to find it. First look for any loose ground wires. First place to check are all ground wires coming off the battery(ies). While doing that, inspect and clean all ground connections. If that doesn't fix the problem, then test ground wire resistances with a digital voltmeter. Should be extremely low resistance on all wires and terminals.

Ayuh,.... Agreed,... Loose, corroded, bad ground connections, could be anywhere, or everywhere,....

Start at the battery, 'n clean 'em to Shiny metal Clean,....
Greasein' 'em, will keep 'em corrosion free,...