Not sure on the fan, my buddy gave it to me along with the tent. So the osculating fan moving air around isn't enough? The doors on the tent are open, if that matters. How low does the RH have to be?
Yes, an oscillating fan should be plenty as long as it's 16+ inches.. 30-40% RH is perfectly fine. And just a little FYI, PM thrives in too low of rh. Where I live my RH is naturally around 5%. My rooms stay around 15-20 percent with no dehumidifier. I fought PM HARD for a year, about 3 years ago. Got infected by some cuts a buddy gave me. Everything in my room was just about perfect, intakes, exhaust, cleanliness, etc etc.. I couldn't get rid of it for the life of me. I tried everything from the moon and back with no luck. I finally caved in and said fuck it. Cleaned everything really well, set off a fungaflor bomb, and treated all veg plants with Eagle 20. I didn't even drench the plants, just a light misting.. I have never seen a spec of PM since then and haven't needed to spray Eagle since then. I will always recommend Eagle if done properly.
Without an intake fan you are running what's called a passive intake. It's just an open intake where air is being sucked into the room from your exhaust fan. That is a perfectly fine way to run everything, just make sure you are getting enough air in through your intakes.