Re: 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?