Be advised, RTV is not resistant to gasoline.

I'd pull it apart, clean it all up, lay the manifold on the heads dry and start checking with feeler gauges all around.

Nice thing about it...you can reach underneath the intake and check for gaps with feeler gauges as well as on top.

If it seems there is an impossible-to-see issue still, you can also reach underneath the intake (without the valley pan)
with a flashlight, in total darkness, and you may be able to detect a gap somewhere by shining the light on it and
observing from different angles.

If something is cut at the wrong angle or what not, you might be able to find it doing this.

You can try this, too.
http://www.hughesengines.com/Upload/TechArticles/INTAKE_MANIFOLD_SEALINGJULY2015.pdf