I'd give OODM a call and tell them what you have and what you want. They have a good reputation and can probably order what you'll need.
Tossed my Coleman about 2 years ago at start of full Monty, covered hole with sheetetal since, recently been prepping and research to be sure everything up there is ready for new dometic unit and have learned a lot.
Had to add reinforcement to roof, original hole was right behind a rib then nothing for 42", sheet metal alone can't support an air conditioner and the sag actually wore a hole through the roof skin . All fixed now.
Had to open up the hole a tiny bit, dometic calls for 14-1/4" pretty easy to do with a jug saw metal blade, ruler and framing square .
Old electrical wiring, old unit only used 120v, there wasn't a 12v
run to the air conditioner, since I have the ceiling panels down I used 2 unused wires I had found earlier. I crimped on extensions to reach the air conditioner area. So there's 120v and 12v
Original ac used the air distribution box for controlling the ac. This would be easiest way to set up the new unit because of the wiring needed for using a wall thermostat with the dometic. For them you have 3 options for controlling the ac;
1. Simple controls on the air distribution box no additional wiring needed except 12v
2. Single zone LCD thermostat, for this you have to have 3 wires running from where the ac will be to where the thermostat will be, this will let you controll the ac only. There were 4 wires original to the trailer that were unused before , they went from near the ac through the walls and came out where the wires for the furnace came out for the furnace thermostat, take a look if they're there you can use them. If you want the SZLCD to control the furnace as well, so you would only need 1 thermostat vs 2, you have to get the 2 blue furnace wires to the ac area, they will run in the overhead in the wire bundle on the curb side getting to them may be possible through the side of the ac opening in the ceiling but would probably be pretty difficult without the panel dropped. Since my pannel was down I was able to get to them , cut them and splice extra length to feed the ac area. So now I've got 4 blue wires in the area , 2 blue in and. 2 blue going to the thermostat as original.
3. Climate control center II, you need this if you get an ac unit with heat pump or heat strips. To use this a phone line wire has to be run from the ac area to where you want the thermostat . One end of the phone line has to have the RJ-11 connector cut off and a new one crimped on reverse wired. If you want furnace control from cccii you need furnace wire as described in 2 above. An alternate to the phone line; if the 4 unused original wires are present as discussed in 2 above, you may be able to use those and crimp them to phone line at either end and connect those ends, they both have 4 wires. There are crimps for small phone line connections like this available at big hardware stores.
There are a few other things like boxing in the perimeter of the 14" hole.
I've done a ton of research to figure the change/upgrade out , hope it helps.
Highly recommend downloading the installation manual for the unit you want and going through it in advance.