Can I use a gasket sealer such as RTV, to fix an air leak where the manifold and heads meet?

Hello,

You can use a sealer, but it will likely be just a temporary patch.
Good chance it will start to leak again. It depends on the size and the location of the leak it might not seal at all and leak immediately after starting the engine.

The best option is to remove the manifold and replace the gasket.

https://www.2carpros.com/articles/replace-intake-manifold-gasket

Please let me know if you have any questions.
Thank you.