Your experience is perfectly normal. Battery will drain because of the parasitic draw. I have my camper plugged in at home 24x7x365. But there are occasions I unplug, specifically when severe thunderstorms are approaching. A couple times I've unplugged, I forgot to plug back in. I discovered that something in my camper will start beeping when my battery reaches a certain low level. First time I heard the beeping, I thought the camper was going to self-destruct. :E
When I finally turned on the lights and saw the lights were very, very, very dim it finally hit me what the beeping was. Plugged back in and the beeping stopped. This happened a couple times. Time frame? about 2 weeks for me!
So now, I've careful to flip the battery kill switch when if I unplug because of bad weather.
I have no clue how long my battery would stay charged with the kill switch active. Never tested that out. But, if there is absolutely no parasetic draw, it would probably hold a charge for quite a long while.
Your experience is perfectly normal!
