[Richard]

I have done a lot of welding on the coach replacing basement floors. I disconnect the batteries, DDEC, and Allison computers. That may be overkill, but they are pricey. Five minutes to disconnect seems like a small chore.

As Simon says, ground clamp close the the weld zone clamped to a clean, bright and shiny piece of steel.

I am assuming something has cracked or broken in the side room structure. Go Slow, short welds, and give the steel plenty of time to cool. If you get too jiggy, the heat will transfer to the aluminum skin and bubble the paint. No bueno.