Probably a confusing reply since this is coming from a different user name (I originally got locked out of the licoricewhip account and created a new handle, but then at work got autologged into licoricewhip and made the post through there on accident). ANYWAYS.
The more I'm reading, the more I think just swapping out the old air conditioner for a new one would be the way to go. For one, I don't even know if the old AC works. I think I could just wire nut on a grounded plug and plug it into a standard 110v outlet to test it, is that right? But the other thing is the original shroud and interior vent / controls are pretty old and brittle and buying replacements would probably be tough, require retrofitting and would negate some of the savings.
I was thinking about going with a Dometic 15,000 btu unit with heat pump. Does this seem like a reasonable option for a 27' trailer, or would I likely need a second AC unit to cool a trailer that large? I don't plan on doing any extreme desert camping for what it's worth.