Yes the switch has to be "On" to charge the batteries (and run the propane detector among other things).
With the newer rigs, Montana is using a intelligent converter with multi-stage technology. This means it won't cook your batteries as older converters would do. So leave the disconnect switch on all the time. It won't hurt anything. The newest Montana's use a Intelli-Power converter. I suspect your 2009 has Xantrac multi-stage converter. You might check the specs or hunt it down. The smart chargers all function pretty much the same. Here is a description of the various stages from the Intelli-Power manual.
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Inteli-Power 4500 Series models can recharge the battery to 90% in 2-3 hours using our patented Charge Wizard technology.
The built-in Charge Wizard automatically selects one of three operating modes: BOOST, NORMAL, STORAGE.
BOOST Mode - Indicated by green LED remaining on. The output voltage is 14.4VDC to rapidly recharge the battery up to 90% of full charge.
NORMAL Mode - When the battery is between 50% and 90% charged, the green LED will flash once per second. When the battery has reached 90% of full charge the green LED will flash 2 - 3 times per second. In this mode the output voltage is 13.6VDC and the converter is safely completing the charge of the battery.
STORAGE Mode - Indicated by green LED flashing every 6 - 8 seconds. In this mode the output voltage has ben lowered to 13.2VDC, the RV battery is fully charged and the converter is maintaining the charge.
http://www.progressivedyn.com/all_in_one_pd4500_1.html
On Edit: Corrected spelling of Xantrac