I guess I don't understand the application you are looking for. Are we talking 80degrees ambient under cooling load, or cold ambient under an 80 degree heating load?
The reason I ask is because your air conditioner unit should be dehumidifying the air by itself, so if you have humidity problems under cooling load, something is wrong.
Now, humidity problems under heating load is a different story. Propane heaters make humidity, so if your humidity is more inside than outside, you may have an exhaust leak from the heater into your ducts. If so, then you have bigger problems to worry about than humidity. If the humidity is more inside than outside, but you do not have an exhaust leak, then you may have condensate or an actual water leak inside your system. Try running the heater for a couple days, on a low-humidity day with the windows open to see if you can dry out whatever the problem is. If you cannot, then there may be water leaking somewhere.
If the humidity inside is the same as outside, but it is warmer inside, so it causes problems, THEN I would do what Joe said and get a couple DRI-Z-AIR units and build a little plenum at the intake and put them in there. There are also smaller units. Your local RV store will be able to give you ideas, too.
I am from dry Spokane, so I always assume humidity problems to be something other than environmental humidity (since we have none). YOur problem may very well be environmental humidity, so an in-duct or in-intake dehumidifier is your best bet.
Where I come from, we spend BIG money to add humidifiers to our HVAC systems so your nose and eyes don't dry out all the time in the winter, because it is so dry. Big difference.
I spent very little money on the cast iron kettle that I put on top of my wood stove and let the boiling water humidify the air. Why pay big money when you can do it cheaper?
PS, the nice thing about DRI-Z-AIR is they do not use any juice to run. THey are completely autonomous and will not add extra drain and strain to your e-system...
