Lukewarm and room temperature both describe moderate heat, but lukewarm implies something cooled when it shouldn't have, carrying a faint complaint. Room temp is neutral and preferred for water. Native speakers feel the difference.
Lukewarm, room temp. What's the difference? They mean a similar temperature, not too hot and not too cold, although lukewarm means a little bit warmer than room temp. But actually, lukewarm sounds a bit more negative to me, especially when talking about food or drinks. It sounds like something was supposed to be warm, but it's cooled down. If I say, "My coffee is lukewarm," I'm probably complaining that the coffee isn't hot enough. That's why it sounds totally natural to say, "I like drinking room temp water," but I would never say, "I like drinking lukewarm water."
