I don't know what is worse in BBQ chasing temps or chasing humidity.
Granted it's interesting to know what the RH is in your cooker but it'll have a negligible effect on your product...for all intents and purposes. I know this doesn't answer your question but adding water to your cooker won't solve your problem with any dry meat. Best advice I could give is learn your cooker, moisture is coming from the meat NOT from the air.