There is a trigger. After every 2 fights without a long rest, the characters will say "oh I'm soooo tired - I should rest."
Which is funny because I will not long rest after easy fights, and I pick a lot of them. I feel like they should change the trigger to something like "I have something to discuss with you at camp," or something that triggers after plot points. Companion dialogue isn't just bonus for camping - it is often the whole reason to actually camp.
I would prefer that it be more than an audio queue regardless of whether it's "I'm tired" or "I need to talk". Like when you have a level up visual clue, you should have a visual reminder when characters have something important to say at camp.