Hi Peter...welcome to the forums and Vintage trailer ownership
I had rot under the front windows on my 75 and near the door. I replaced the entire front section of plywood. I did in by cutting a sheet in half on a bevel, then used a backing plate under the joint. I covered the joint in glue and screwed it about 4" on center. The reason I cut the plywood in half was to make it easier to get back in under the C-channel at the front. You are probably going to need to pull the belly pan and banana wraps too and check your outriggers and frame for rust. Good news is that the outriggers are still available from an Airstream Dealer and they aren't horribly expensive. Or you can fabricate them out of 18 gauge steel.