The story I was given back in the day (by Fleetwood engineering staff), was that the luan that had already been in use for a few years, was showing that there was a problem with it delaminating very easily, causing issues that were BIG bucks to repair. Replacement of entire sides were often required under warranty. In other words, it was already developing a bad reputation very quickly.
In an attempt to rectify (and save manf's huge money converting construction to a different type), there was a chemical introduced in the luan (which is just a thin plywood basically) during the bonding process as the layers were glued together. This chemical was SUPPOSED to prevent further issues. It soon became clear that not only did the chemical not stop the delaminating, it often caused the pitting/corrosion we're discussing here.
To my knowledge, no other changes were made in the bonding process used to make the sides and tops. If somebody was using a new cement to bond the alum. to the luan that would be news to me, but that's certainly possible.
Today, if regular treated plywood (the green tinted stuff) is used to re-deck a pontoon boat, there is a good chance that any alum. in direct contact with that treated plywood will eventually show signs of corrosion, even in fresh water only environments. Marine plywood uses a totally different (and apparently much more expensive) waterproofing process to prevent that exact issue. For whatever that's worth..... -Al
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake