Adding a second battery on a truck camper

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Hi, Just a question hopefully someone can help me out.
I just bought an older camper. It runs the running lights, three overhead lights, fan for the furnace, and other small electrical devices off of the 7 pin plug from my truck. I would really like to add a second battery to the unit. I was thinking that I might place a deep cycle battery in the box beside the camper and have that 12v wire go to a battery selector in the truck, to which the 12V wire that comes from the 7 pin plug (which is used for the power now) would be attached to aswell. This way I can choose to run off of the deep cycle 2nd battery without worry of killing the battery in my truck, then if needed I can switch to my truck if the deep cycle battery dies/gets low. Also (i'm not sure about this) i could select both batteries while my truck is running to charge the deep cycle battery.
Thanks for any info