I've got one that holds about a gallon and a half that I use for my FW here in Florida. I have to dump it twice a week in the summer and I found that I also need to use a fan to circulate the air in order for it to work correctly. I had it setup in the kitchen area and last fall I found that the pillows in the bedroom mildewed.
I keep the bathroom door partially opened and run a household fan on a timer. I keep the dehumidifier set on "medium" and dump it twice a week. It seems to control the mildew and smells in the RV.
About once a month I dump a little of the water from the dehumidifier down each of the drains in the RV to help keep smells from the holding tanks from coming up the drains.