High wattage 12 to 5 V
Look for DC to DC converters that have a clock sync and/or parallelizable: 

Here is a list from analog of high current DC to DC converters. None of them got to the 300A you require, but some go to from 60A to 90A, in which you could parallel three or four of them. However DC to DC converters can be difficult to size components correctly and required a lot of time reading in the datasheet and a certain amount of skill which is not for everyone (especially me, when I got the wrong mosfets and the converter doesn't work on it's first cut). 


Source: https://www.analog.com/en/products/ltc3884.html

There are also high current LED drivers like the LT3761 which can handle 100A:



It may be better to follow Jack Creasey's comment and buy off the shelf ones. Sometimes they can be difficult to parallel if the voltage outputs are not matched well. 

It would also be better to send the 12V (or higher) across the slip ring to reduce losses from the contacts and have the 5V conversion after the slip ring to reduce I^2*R losses from the slip ring contacts.