When a suit has been agreed (bid and raised, or raised by inference--such as a splinter raise or conventional raise), that suit is trump for RKC. If there was no suit agreement, then 4NT is RKC for the last naturally-bid suit. Here are all the details.
Responding to RKC with a void: If the location of the void will be known (from the previous bidding), show it as follows: 5NT=2 Keycards and a void; 6-level = a void and the same # of keycards as the 5-level response (so 6=1 or 4). If partner won't know where the void is, just ignore it (answer on the 5-level your # of keycards).
King-asking: 5NT guarantees all the keycards and asks for # of kings using plain old Blackwood (6=0 or 4, 6=1, etc.)
If they interfere after 4NT, use DOPI: Double = 0-3 and Pass = 1-4 and bidding the next step= 2 keycards.
Gerber is used only when the last bid was 1NT or 2NT (plain Gerber 0/4, 1, 2, 3).
Note: There are many fancy options here, including kickback and specific kings. These require many hours of partnership work, which goes against the grain of LC Standard (to be used quickly and effectively by new partners as a starting point).