Powering multiple devices from a single power supply
Search for "DC-DC converter" look for "12V to 5V". 

The popular 'selling websites' should have something which will step down 12V to 5V efficiently. 

They sometimes say 'switch mode'. 

Avoid 'linear' which is the parts you have been using, Linear voltage regulators will need to convert the power of the 7V voltage drop, and 700mA for the R-Pi to heat, which is about 5W. A waste of energy and battery charge.

It may be much better to use two DC-DC converters, one for the R-Pi, and one for the servos. 

Some servos (which look like the ones you link to) take a huge stall current, and might dip the R-Pi voltage enough to make it erratic. The R-Pi should be okay with 1A