The treated wood in your trailer probably contained chromium copper arsenate (CCA), which used to be the standard wood preservative. It, and almost every other newer wood preservative, contains copper of some type. That copper in contact with the aluminum was likely the source of your trouble.
Aluminum and copper are some distance apart on the
galvanic corrosion table, indicating a good chance that corrosion will occur if conditions are right.
The issue with PT wood corroding aluminum is well known in the building trades: Aluminum nails and fittings aren't used with pressure treated wood and any aluminum flashing needs a barrier between the wood and the AL to prevent corrosion.
I guess the builder of the trailer just didn't care.
From what I've seen, builders of many RV's have great faith in the ability of caulk to keep water out. In my opinion, any design that depends on caulk as the primary/only water barrier between surfaces that are subject to vibration, exposure to UV light, and to large changes in temperature is a design for failure.