I don't know of a commercially made solution, but a DIY solution would be simple here. A dehumidifier in a sealed box with two duct flanges for intake/exhaust, add your in-line fan, done.
Parts: piece of plywood, a 2x2, a tube of calk, two flanges, screws, a fan, ducts, and your choice of dehumidifier.
You will need a saw, a drill, and a brain to accomplish this though.