The job is best done by removing the interior furnishings and interior skins in the area of the repair, then removing the rivets holding the outer skin and window frame in place, and then buck riveting, using solid rivets, the new segment and window in place. This is a big job-plan on a couple of days. You will also need a pneumatic riveter and bucking tools, which will run you a couple hundred dollars, assuming you have a compressor already.
Alternatively, you could leave the interior intact, drill out the rivets from the outside, and replace the skin and window using Olympic rivets, which are a kind of blind or "pop" rivet with a head that can be dressed so that at the end of the job it looks just like a bucked rivet. This is the approach that many professional shops take, and although considered by many to be not as good as the first option mentioned above, it is acceptable. This approach will take half as long as the first option. Allowing for the learning curve, I would still guess you could do it in a day. As far as special tools go, it would be good to have a pneumatic pop riveting tool--you can get one at Harbor Freight for $30 or so. The Olympic rivets are pricey, and you will need to rent/borrow/buy the rivet head shaving tool, which is quite pricey.
Those wing sindows in 1969 are hard to come by and much more expensive than the ones used in the 70's. Make sure you are getting the correct window for your year. Also, if the glass is still intact, you can rebuild these windows as a single pane, if that is your goal.
The lower corner skin is a 3D formed piece of aluminum, not just a flat sheet that wraps around the corner. You can buy a new part that has the same shape as the one on your trailer, but the alloy will almost certainly not match the rest of your trailer. With this in mind, you may want to investigate ways to repair the dent (ie., with heat and a "dent roller", or finding someone with an English Wheel who can work the dent out).