I had the exact same issue with my 10x12 greenhouse and I upped the air flow and did a deeper gravel bed (5-6"). The deeper gravel bed was a MUST and I'm sorry I ever tried to "cheat" and stretch on that score.
As for air flow, I simply cut a square in the bottom of the back panel and stuck a box fan in there. I took the plastic panel I cut and stuck it back in in such a way that there's now a little "roof" over it. I also have four fans in each corner of the greenhouse that blow air in a circle pattern, and then an exhaust window fan up top. It doesn't seem to really matter where I position the fans or anything, as long as there's a "flow" it can be extremely soaking wet in there, and it doesn't feel "damp" at all and my plants, seedlings, cuttings, orchids, etc, are all thriving and happy.
