Evaporative coolers only work when the humidity is well below 50%. This is why they are popular in the derert climes. In addition, all of the water that they use is pumped back inside your trailer and can lead to mold and other moisture damage if you are not careful.
We use a small Coleman LP Heater or a 1200 watt electric heater when hooked up. We also have a furnace
, but it will cook you out of a Hunter in about 5 minutes when turned on.