Any reason to keep vintage Duo-Therm rooftop unit?

If it was me, and I knew I was replacing the skin, I would remove it. Sure it works, but it's old (can break any time) and the roofs on these trailers were never designed to carry the weight of the unit, anyway.

Going down the road mile after mile may eventually cause ceiling sag which you don't want, especially after you did all this work.

Maybe consider a Fantastic Fan or a Maxxair roof vent. You can get them with a vent cover or just the lid.

I have never used one, but I have read good things about them. Maybe install one of these, see how it works for you. and if it's not enough cooling/venting, then add the window AC as you mentioned temporarily until the mini split is installed, but even those can be weighty. Sure people do it, but they can create leak issues.

Some people have installed the window AC unit on a slider tray that slides out from the storage compartment door when in use, but then it can mostly cools your feet.
I have read that a carefully placed fan inside the trailer can mix the air.

Cheers

Rod

Great, thanks for the confirmation. Since I'm going to end up installing solar panels over the opening, I won't have room for a vent there. But the idea for the solar is to be able to power the new A/C with no battery drain whenever the sun is shining so the vent should be superfluous.