During the summer when we aren't using our 21ft TT we hang a DampRid moisture absorbing bag in the trailer. Seems to do an adequate job of keeping moisture under control.
I winterize the trailer in the winter, no need to keep it heated. Especially since it is not a 4 season trailer (uninsulated), heating it wouldn't accomplish much as the pipes and tanks under the trailer would freeze anyways. I hang a dry bag in the TT in the winter also, but it many times will last all winter before filling up.
Our trailer is not very big, so this works for us.