I power my Galaxy Tab 2 10.1 from a Blue Sea USB outlet in my 4Runner. It provides enough power to run it at full brightness. The USB outlet provides 2.1 amps (10 watts): http://www.sierraexpeditions.com/index.php?l=product_detail&p=8670
I use the USGS maps (cached) with Backcountry Navigator and I can zoom down to about 500' full screen (per the scale, 20X zoom by the buttons). It does get pretty pixelly, since the tiles are bitmaps (I think they are actually PDFs for something).
I plan to run my in my FJ60, which should help with the power... should also help recharge when I'm out of the rig, on those foot-bases critter pursuits!