solved long ago.
Get yourself some regulated 12V buck-boost supplies. I use a Logic Supply Intelligent DC-DC Converter; good for 10 amps. Then get some 5 VDC regulators from mouser; a Texas Instruments PT78HT205V works nicely.
Now you're ready to rock. Plug a Roku into the 5VDC regulator, a TV into the Roku, and you're almost there.
Get an OpenWRT compatible wifi access point. A Buffalo WZR-HP-AG300HR has more horsepower than you need. Plug a hard drive into it, set it up. Install lighttpd and roconnect, install the ro-cnnect channel on the roku, and you're good to go for movies.
Now install some of the other channels; you can play movies from a usb stick, and there are some music channels.
Set up the wifi access point to serve up the movies and music; now you can stream from your van to your iPod or whatever.
And the whole thing draws about 3 amps, not enough to worry about.