Das Lagerfeuer, in dem ich meine Marshmallows brate, ist sehr heiß.
Quote: "Am I correct in saying that the article used in this relative clause is 'dem' ...?"
Not exactly. Here, dem is not an article but a relative pronoun. It indicates that the content of the relative clause (Marshmellows braten) is referred back to the last noun in the preceding clause (Lagerfeuer). It is in dative case (dem instead of das), because the Lagerfeuer is the location where the action of roasting takes place.
Notice that the noun referred to does not need to be the subject. It can also be an object, as in
Ich(subject) fotografiere das Lagerfeuer(object), in dem ich meine Marshmellows brate.