Just a thought, can you leave your rh meter outside the tent so you get an ambient reading?
Are you watering to run off? I water my girls in the bath, leave them 30 minutes so if there is any runoff it's not sitting in the tent. Although now I'm growing organically I try not to get any runoff at all.
Lastly, I water on alternate days so there's less moist soil sitting in my tent, if that makes sense.