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I suspect you have a diesel engine. The 2 batteries are needed to start the engine, it takes alot more CCA (cold cranking amps) to start a diesel engine.
I own an old truck camper and am bit frustrated with Ford, for not leaving any room under the hood of that camper rated F350 S.D. 6.0L diesel. I do not even think there is enough space to mount a battery isolator. Just looked at the 7 conductor trailer hook up and even that was not really capable of handling a deep cycle. The battery charge conductor is apparently fused at 20 amp and their is isolator relay in the fuse box. Looks like it will be major effort, with #8 wire and battery isolator and a battery box to support the truck camper.
Nope, it's for the diesel engine. The engine needs more cranking amps and when it is cold they run the glow plugs to help warm the air before the engine will crank over.
Usually, when you have seperate battery for a camper it is isolated from your main batteries or you have a switch to isolate it. That way you use a lot of juice from your camper battery but not drain the ones needed to start your truck.