You can run software for home use, then different software for car use.
For I have a computer system in my car also, so I run Frodo Player which integrates GPS navigation well. For $70 dollars you can get a GPS receiver and add that to your lil portable project and always know where you are.
www.mp3car.comwww.skjukebox.comwww.frodoplayer.comwww.usglobalsat.comAt home right now I run SKJukebox on my Regular PC on a 15" Elo LCD Touch on my Video cards secondary output.
Also you might consider getting a smaller Touchscreen or so since it's gonna be portable. Also I recommend a Mini ITX board with nice mini PS like a PICO and that way you can run it off the 12 volt DC Power brick at home off 110volts AC, and then rig up a 12v Ciggerette adapter for your car/rv.
Build a small wooden box with a fan to house the LCD the board and the rest of the components and add a handle to it, and wala. Your portable Jukebox/CarPC Navigation system.
This will run you about $1000 including the touchscreen though, so be ready to spend some money.
You can always fall back on your iTunes idea on this setup too...