No matter which position the OEM switch is in, there will be some "parasitic drain" on the battery. RVIA (The industry organization that sets standards for all manufacturers) requires that the Propane detector, CO (carbon monoxide) detector and the smoke/fire detectors "always be powered on", so even with the switch in the off position, those will "drain the battery". Additionally, the "backlight and memory circuit" for the stereo will always be powered, so another "added battery drain". You may have others as well.
The ONLY way to eliminate all drains, and assure your batteries are "somewhat charged" after a month of storage, is to disconnect the battery from the system. You can do that with another battery cutoff switch on one of the battery cables (at the battery) or by disconnecting the battery cable, physically removing it from the battery post.
There is an "ages old" debate about whether to install the switch on the positive or the negative cable and that extends to whether to remove/disconnect the positive or negative battery cable (if you choose not to install an additional battery cutoff switch)... Either way, unless you physically "isolate" the battery, it will be completely discharged in a week or so. That "complete discharge" will definitely shorten the life of your battery, and is, for most manufacturers, considered battery abuse and may not be covered under the battery warranty. Repeated, full discharge is one of the most damaging conditions for any lead acid battery.
ADDED: Remember, when towing, the break-away switch, which applies power to your trailer brakes in the event of a trailer separation, is only powered by the trailer battery, so there "must" be a charged battery in the trailer when towing, so don't forget to turn on any cutoff switches and/or reconnect the battery before hitching and towing. For most of us, with an electric tongue jack/landing gear, that is a necessity, but for some, with a "crank up tongue jack" it can be forgotten and can lead to an unsafe towing condition.....
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS