I'm not trying to sell anything, but I did some web sniffing and found this link. THEY ARE trying to sell something, but I got quite a bit of "optimum humidity" information from reading this link.
Here's a quote "Another product to invest in is a hygrometer. This useful utility measures humidity so you know exactly how high it's actually getting.
Recommended humidity levels are between30% and 50%. If humidity levels in your RV are exceeding this point, then you definitely need a dehumidifier".
Humidity Levels in Stored RVsAlso, the guy we bought our TT from told us that EVERY MORNING when they got up *it was a family of 5 sleeping in it* they took those blue shop towels and wiped down all the interior windows. I am AMAZED at how much water we get off the windows when we do that.