RTV to seal mufflers is a recipe for disaster. First, you don't need it. Second, if you use anything more than the very thinnest smear, it can/will fleck off and get sucked into the combustion chabmer during the intake portion of the stroke. If that happens it will snuff the glow plug immedately.

A properly faced muffler/engine joint doesn't need any gasket or RTV. If the surfaces aren't flat, then a thin smear of 5min epoxy will seal it without the issues of RTV