If the humidity is really high, adding ventilation may do very little to control mildew, lowering the humidity is the answer.
We have an upstairs bathroom with a window that does not open and we live in the Willamette Valley where mildew prospers handsomely (unabated, it appears in the shower overnight).
This little 14" tall baby (Ivation GDM30) is what I have and suggest. It is designed to work within 2200 cubic feet (almost equivalent to a 39' RV) and pulls about 20 ounces of water per day (86 degrees and 80% humidity) and tank capacity is 50 ounces. Want a bigger tank? Then purchase a bigger unit.
https://www.ivationproducts.com/products/ivagdm30-midium-thermo-electric-dehumidifier#About every 10-20 days we empty the reservoir and have it on a timer; off at nighttime and on in the morning. WE HAVE ZERO MILDEW ISSUES.
And, if you want to run it at 12VDC, then eliminate the 120 VAC power adapter.
Tim & Sue
Hershey (Sheltie)
2005 F150 4x4 Lariat 5.4L 3.73 Please buy a Hybrid...I need your gas for my 35.7 gallon tank!
2000 Nash 19B...comfortably pimped with a real Queen Size Bed