Camper 12v to USB charger box (phone charger)

I recently bought an older camper and we often go boondock camping (camping with no hookups). I wanted to add an option to charge anything via a USB connection off of the campers battery. I wanted something sleek but functional. I built a 2 part box that screws to the underside of the cabinets. I found a 2 plug 12v usb plug with a built in voltage meter on amazon (https://www.amazon.com/gp/product/B01M0GR0FQ/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1) and i added a hole for a 12v push b