I would say normal
When you put the power to it, the convertor immediately looked at the battery and said, "Hey, time to charge..." Charging circuit uses transformer that calls for the cooling fan to come on. Nothing to worry about.
While the battery was sitting in the trailer, apparently doing nothing, there are "hidden" users of a battery's stored supply of "juice". Most notorious is the LP leak detector...draws a fair amount of current for such a small device and, for safety reasons, there isn't any way of turning it off. Other users of current, even when a device is off would be your Jensen sound system, the 12v signal amplifier for the Sensar TV antenna, etc. Lots of things to draw a battery down.