Most efficient way to power a 5 V circuit from a 14.8 V battery
There are dozens of small, inexpensive DC-DC converters available on Ebay, Amazon, etc.  They are based on industry-standard converter chips and are quite convenient to use. You can find versions with fixed output voltages, as well as those with user-adjustable output voltages.

I would probably use TWO of these converters. One for the display (at 12V) and one for the RasPi (at 5V). The very first hit I found on Ebay shows that you can buy 10 of them for less than US$12 with free shipping.  They are good for 2A, but will do 3A if you add a heat-sink to the regulator.