Aluminum rotting on Lance 915

Aluminum can (and does, as you have found out) corrode.

What you see is likely a result of this: https://en.m.wikipedia.org/wiki/Galvanic_corrosion
I would not be surprised if the frame behind the corroded area is wet.

Was part of the camper frame rebuilt in that area? With PT wood? The chemicals in it can greatly accelerate corrosion.
How big is the affected area?
What year is your camper?

Aluminium, just like copper does rust, but the rust make protective coating, that prevents future corrosion. What you see as green stuff on copper, is having the same color what base material on aluminium, therefore you can't tell the difference.
Still when the corrosive coating gets rubbed off, or with heavy road salt accumulation it will corrode. I used to have aluminium body motorhome, where 1/8" plates on fenders had corrosion holes.
To some degree you can use Bondo to patch it, or make bigger repair with fiberglass.

Agree.

Galvanic corrosion will happen on these when the aluminum touches the steel and there is moisture (water or high humidity from a leak).

You will need to prep and paint the metals or isolate them from each other, both electrically and atmospherically. The dis-similar metals do not have to touch, just that they are close and the water vapor can facilitate the process is enough. Im not sure the exact process the water vapor does.

Just making it where the metals do not touch will be the biggest if you have to choose just one fix. A plastic shim maybe…

I had an aluminum roof do this and it was from the steel frame and a leak.

Several bubbles in the aluminum sides of the Sahara where it meets the stainless steel basement doors. Once you start picking at it, the paint needs to be dealt with. Not a great prospect on a 24 year old rig.

The one area I did pick at I wire brushed and then hit it with a coat of paste wax. That seemed to stop the corrosion. If you camper is white the contrast wouldn't be noticeable.

I just inspected a motor home that had a handicap access door installed. It was aluminum with steel parts attach. The entire bottom of the door was corroded.

Per my experience it takes salt to make electro corrosion.
I have aluminium steps, with SS pins, about 5 years of use in no-salt states and not even a sign of corrosion.
Same was on my boat, where aluminium railing with SS fasteners was perfect for 10 years of lake use, till I took it to the Ocean and than 3 rides made more damages than 10 years on the lakes.

On the plus side, most people find the material is easier to work with. BTW, deicer is about as bad as saltwater to damage aluminum or rust most anything.