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I was playing the incoming scenerio when a question arose. If your opponent destroys the objective on your side of the table what happens to the marker? Does it just stay there as an immovable object? Does it become a wreck marker (difficult terrain)? Or, does it just disappear?
A rule citation from an official source would also be appreciated.

Disappears. I don't think the steamroller pack states this obviously but you can work it out from the following:

Steamroller pack says it is a friendly model to both armies.
Models, when they take more wounds than they have go through the disabled, boxed and destroyed (DBD) process.
Warjacks have special rules stating they leave wreck markers.
Therefore anything that doesn't state it stays in play after going through DBD (see knight exemplar scenechal) or leaves a wreck marker (see warjacks) will be removed like any other model.

Your logic looks good. That's kind of what we thought and went with.