Opening slide

I'll move my slide out while the truck is running so I'll know if it (the slide) will affect my side-to-side level.

Like others say, the truck's charger circuit is pretty weak.

And 30A for 15 seconds of slide operation is 0.125 Ah ??

But then there's the electric Tongue Jack, electric stab. jacks, electric awning. They all pretty much depend upon the batteries for operation. But they are also all short duration.

Best thing is to have a well charged, substantial battery bank.
Then you don't have to worry about it, especially with solar panels, or when plugged into 120V.

While I have access to electrical I plug in first and then operate the slides. But when I don't I move both my slides on battery power with no problem. (I have a Group 29 Marine deep cycle).

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

I also prefer to put mine out while still hooked up to the running truck. Saves the batteries charge.

Another thing you should do is open the entry door before opening and closing slide(s). It allows air to flow in/out and displace more efficiently which provides less strain on the slide systems.

I perfer to extend it while hooked up to a running truck, but only to save the AH's for something else.

It probably isn't a big deal either way.

Yes - but the world won't end if you don't.