Perhaps relevant for your initial problem: Can you charge the tablet form a power jack? If so you can try to use a combined notebook power supply with additional USB power out. E.g. http://www.conrad.biz/ce/de/product/515004/VOLTCRAFT-NPS-90-USB-Notebook-Netzteil-Laptop-Netzteil-BASIC-LINE-12V14V15V16V18V185V19V195V20V21V22V-/ So you can drive the tablet by power jack and the Arduino via USB - if 5 V/1 A via USB fits your need.