someone else mentioned using bullet connectors in another thread for nearly the same purpose for inline splices
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to meet all of your criteria I can see price on the pc end adding up. You should be able to lengthen battery life by having an SSD in whatever you choose to use but they get pricey quick. Battery runtime is usually advertised with just about everything draining turned off (think things like optical drives, usb, and network connections). For 6 hours of show running you'll probably want a notebook with a spare battery or two charged and ready to swap out. They do make tablets that run full windows 7 for under $600, but the battery life isn't to the point you need and you can't swap out the battery so you'd still need multiples.
One option that might be worth looking into is what they used to call a carputer. I haven't kept up with the tech, but basicly it's a PC with a special power supply made to run off the already 12v vehicle electrical system and using small automotive lcd's as a display. The guys that build them usually use the ultra small form factor boards and such. No idea on the drain it would put on your big batteries though. My thought is if it's feasible to use you might just need to expand your existing battery bank to power it.
Most straightforward thought is some kind of quiet generator (oxymoron I know) muffled up etc.
