Switch is labeled "BATTERY DISCONNECT" and yes as labeled key switch should be in the ON position. I will see if I have a picture. Think you said you have a 2011 300mp would be surprised if yours was in a different location. Well after reading others comments may be not
You keep bringing up this 12V transformer, not sure what you are referring to?? In our rig 12V DC comes from our batteries (no shore power) or the Converter when connected to shore power. When connected to shore power and the converter working and Battery Disconnect Switch in on position as labeled, the converter will charge the batteries. If Battery Disconnect in off position and converter working 12V DC will be supplied to the rig but the batteries will not be charged.
Hope this helps, there have been a few posts addressing the position of the Battery Disconnect Switch. Can use search to find previous posts
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2013 RAPTOR 300MP w/Rear Patio Deck NO Folding Side Ladder
2013 Silverado 3500HD LTZ CC LB 4X4 DRW
Duramax 6.6L Turbo Diesel; Allison Trans
Reese 20K; Carlisle Radial Trail RH/HD; TPMS (12 Tires)
Veteran
PSU (GO LIONS)