A small AC running constantly is a great dehumidifier.
My 12x20 reloading shack is very well insulated and sealed. I have a small window AC that I keep the thermostat set to a comfortable level. When the outside temp is not warm enough to cause it to run, humidity is usually not much of a problem. I have a temp/humidity meter in the shack, and I've never seen the humidity over 60%, most of the time during the hot summer it stays around 45%.
In the winter, I have an oil filled radiator on a thermostat that I keep set to about 60 degrees. If you take humid air and warm it up, it lowers the relative humidity. As long as you don't have a moisture source, the humidity stays down.