So, I've got a guy around the corner who mills, slabs, dries ... etc. He also uses a "greenhouse", to dry wood, but there's a huge difference between how he does it, and what you describe. One thing occurs, when air is heated... relative humidity drops.. significantly.. It's kinda like when you run the defroster in your car ( if you live somewhere where there's Winter). It would seem, that it doesn't matter what method is used to heat the air, the drop in relative humidity seems to be a strict function of the change of temperature. It's like a house, in the Winter, ( once again, if you live where there's Winter). It doesn't matter the method of heating ( much), whether it's electric, natural gas, wood burning fireplace, etc.. It seems that if you raise... ( let me pick numbers outta my %utt..) , air at 90 % humidity, at 20 degrees, Fahrenheit, and heat it to 80 degrees Fahrenheit, it'd be down into the 20 % relative humidity range. That's why you need a humidifier in the Winter... stops your nose from bleeding.I sort of duplicated this with my greenhouse, minus the dehumidifier, which I'm not sure how much help would be being outdoors and in ambient Temps. I did keep both ends of the green house open for air flow. Lumber can only dry out to the point of its surroundings....I suppose the dehumidifier would hasten the process to a point but not likely enough to prevent the boards from twisting after 2+ years of drying.
My neighbour uses jack for air flow.. his objective is to get the highest temp that he can in the greenhouse ( drying chamber), that he can. Swathed in dark plastic, with limited air ingress and egress, he uses only a small solar powered venting fan to draw out the humidity from what comes out of the wood. He says, that on a good sunny day, ( North Bay Ontario Canada area), it'll break 160 F in the "greenhouse", at an ambient temperature of 70 F.
You might want to slow down the air transfer on yours and see what happens. `You want to draw moist air out.. not bring it in.. Or, at least "condition", the air to the point that you want for moisture transfer.
Again.. I'm no pro at this.. at all.. But, this is what I've seen.. and it works..