Seeing Materials in UV/Image Editor and Exporting

I posted on here earlier about another issue I’ve had with textures/materials for this project. Basically I’m trying to get all of the materials onto one PNG in order to put this model into XPlane.

The problem I’m having now is that I don’t seem to be able to get an image of all the current materials in the UV/Image Editor, all I get is a grey background. I can open up individual materials in the viewer, but then it places ALL of the UVs over the one picture/material. Any idea how I can solve this problem? And, also, how do I export the texture/materials as a PNG when I’m done?