I know of a few boats with dual monitors. They've all worked out fine. A couple are simply using a VGA AB switch to manually switch between their chart table and the pilot house. Another has a waterproof, sunlight readable monitor
at the helm
and an office monitor
at the nav table. He feeds them from a DVI and VGA output on his computer. These are all using SFF, 12 vdc computers
but it should work out the same with a laptop
Keep your cable run under 15 feet and use "high quality" VGA cable unless you plan to use a VGA amplifier. The actual distance you can place your 2nd monitor will depend on the strength of the signal from the graphics section of your laptop. You will get ghosting when the cable gets too long.
Most office grade monitors won't provide adequate brightness for use in direct sunlight although they can usually be used in a shaded location such as a pilot house or under a dodger
There shouldn't be any problems running the monitor off an inverter
but you can get 12 vdc monitors that can be connected directly to your 12 volt system.
I'm not sure what kind of interference
you are concerned about but if you run your video cable parallel to your SSB antenna
cable you will probably get interference when you transmit.