First please don't let a trailer cause you both this much stress.
second If you and your spouse are capable and willing insulating a trailer can be done
lots of screws and boards to remove but can be done.
third I have seen AC's mounted several ways some in a back wall, some cut through the front
and others like barbs mounted on slides that slides thru a door built into the side of her CT.
Breaker box is a metal box not unlike the one in your house that houses the electrical breaker devices that protects your appliances, stove, furnace, lights ect.
I have never heard the power cord that goes from the breaker box to the campground electrical pedestal called a "lifeline" but it sounds like that is what they are talking about.
Power cords are usually connected to the breaker box one of two ways direct wired or pluged into a receptacle that's wired into the breaker box.
the difference between the two is the direct wired one has to be stowed by pushed it through a covered opening into the CT,
while the removable cord can be unpluged and stowed in any part of the CT like under a bench or in a cabinet.
again don't get stressed over a CT lifes to short as it is
don't worry, be happy
Pete