A USB tuner is a great idea, but don't forget the antenna too.
There don't seem to be too many options.
http://www.amazon.com/Axion-AXN-8701.../dp/B001FWYLLG
- Doesn't say how long the battery lasts.
- Looks the same as one that Best Buy is selling
http://www.amazon.com/Haier-HLT71-7-.../dp/B001E78UQY
- 2.5 hours of battery life
An ad on amazon.com linked to
http://www.lcddigital.tv/, which has a few models listed there along with a battery pack that will run one of their TV's for over 12 hours. (I know nothing about this site, could be a scam for all I know, I just thought it was an interesting option)