I recently had this problem with my Legacy. Just couldn't get my rocker cover to seal. RTV, or not.
Took it to my local mechanic, after failing 3 times myself (lol). They used an original Subaru gasket (not the Permaseal ones from Partmaster), and RTV in the normal places (plastic half moon cam seals, and in the 90° corners over the front cam caps), and it seals mint. Apparently, original Subaru ones are like 1mm thicker, which makes all the difference.
Went to Owens Suspension & Brakes Specialists in Whangarei. They've been forever excellent to me.