Thanks for the replies, after some further reading online I thing the more appropriate term is Galvanic Corrosion.
Once the trailer is put back together there is no access to the Hold down plate as it is covered by the outside and inside skins. The hold down plate is steel sandwiched between the aluminum skin and the aluminum C channel. There is also an aluminum filler panel which is sandwiched between the hold down plate and the steel rear frame cross member.
Since the trailer moves I worry that most coatings will rub off over time, leaving aluminum in contact with steel.
Mustang restorer's often use Truck bed-liner to coat items that rub such as the fuel tank straps and the underside of the trunk pan which contacts the top of the fuel tank. I may use this on the hold down plate, and apply a coat of POR15 to the inside of the aluminum skin and C channel to insulate the metals from each other. Hopefully a spot of Tempro on the rivets will be enough to insulate the aluminum rivets from the steel plate and keep water out. Once the plate is installed, before I re-install the inner skin, I may spray the area with RustCheck rust proofing oil to try and keep moisture off.
My concerns are probably unwarranted since the trailer survived for 36 years without any of this.