Slide out are typically wired directly to battery due to high amp draw (30A)
12V DC line from truck can't supply that amperage due to length/size of wire.
Plugged into AC power source....converter can keep up but battery is doing the supplying
Good healthy maintained battery is what works best.
Don't know that door needs to be open.........as soon as slides move there is a big gaping hole in side of rv as slide is in transition.
They only seal when fully retracted or fully extended....nothing in between