The statement "truth is relative" is also relative then, because youre saying that you know something concrete about truth, that it is relative. Yet that in itself is not a relative statement. So if you want to stick to you opinion of truth being relative, then you have to acknowledge that your opinion on truth is also a relative one, meaning that it is not true. Thus if its not true, then truth must not be relative. Simple.
I know youre giving examples like "Don't leave your room" and saying its truth. But that doesn;t even relate to this topic. Truth is something like, the Air is made of oxygen and other gasses. Sure we have names for them which may be different in another country, but the fact remains the same, air is made up of those elements.
Sure we may call Red, Red, but even if we did call it blue, it would not be a different colour, it would just have a different name. So those arguments youre giving are not relevant to this topic.
What youre trying to say is for example that Murder is not ok in our culture, but in others it may be ok. But the truth is, murder is never ok, what right does someone have to take the life of another, who cares what culture you live in, each individual is still a full human being, and even if their culture permits murder, doesn't make it right.