Rust between walls

Thanks for the replies. Replace and silicone--------BTW I use SS screws on occasion.

More questions later on Electric after I take a few pictures.

What David said. if i pull a screw and it is rusty. The hole gets filled with silicon and a new screw gets installed.

If you camp near salt water no place is safe. The top running light on my 5'ver dislodged when I hit it with a wash brush. Upon closer inspections the screws rusted completely through.

Now a running light is sealed to the rig with a rubber gasket, then in this case a slight caulk on the top and sides. It is designed to take on driving rain while going 70mph into it. No way should that have rusted, but it did.

Most likely water wicked in from the screws. Use dicor non sag or proflex and fill screw holes and reinstall screws or replace with stainless.

If you camp in cold temps and don't adequately vent the moisture to the exterior, it will migrate to the exterior skin of the outside walls and to the underside of the roof deck and condense into water. In cold weather you should have the roof vents open a little and crack a window or two. Some use a dehumidifier. Sadly, RVs aren't built properly compared to a house (or any type of occupied building). The vapor barrier in an RV is the exterior skin where in a house it's on the inside of the wall assembly and stops moisture from getting through walls.

Otherwise if not the above, maybe you have had water leaking through the wall due to failed caulking? Sometimes water intrusion can be from an area some distance away.