There is no need for that switch. All you have to do is disconnect the negative post on the battery when storing. It sounds like there is a problem with the switch. If so get rid of it and do above.
Check you battery and to make sure it is not defective. Your battery will charge two different ways. It will charge when you are connected to shore power. It will also charge when your 7 pin connection is hooked to your truck and it is running. However to do that you must have a fuse and relay installed in your truck that does that. You will find the location in your truck manual. For some reason Ford sells the the truck with the fuse and relay not installed. Mine was in a bag in the glove box.
I learned the hard way after battery went dead after storing for just three weeks. Luckily I strore it at an RV place and they informed me.