Sounds like there is a connection between the truck and MH batteries.
So the radio and backup camera etc should be powered from the MH batteries. Do you still have the original radio in the dash. Cause that's 1 point where both battery ,+ leads could be connected together. I'm just guessing, off the top of my head.
Keep in mind when your disconnecting batteries pull both+,and- cables. Or if they are under load you will be trying to read current through the meter by disconnecting only ,1 cable.
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