Difference between Ducted and Non Ducted when buying AC?

Besides the obvious of one being ducted or non ducted I am trying to decide where to buy a Ducted A/C unit if I end up having to replace mine. When I call Camping World or someone like that to get prices for a new roof A/C unit when I tell them mine is ducted they always tell me that's a special order. What is the difference between the units? I realize one just blows straight down and one blows down and around the ducts but what makes them special order? I would think any RV built in the last 10 years will have a ducted system so why would they not stock a Ducted A/C?

I am looking at replacing my Dometic Duo Therm Model 57915.541 out of my 2007 Georgetown 351DS if I cannot figure out what's wrong with it in the next few days. Also wondering if this may be a heat pump also as the wall thermostat for this unit runs the Heat in the Motorhome also.

When I am looking for a replacement unit what specifically do I need to ask for? Can I just go off this model number? I would assume it has been upgraded over the years. Plus I would like to probably go to a 15000 unit instead of another 13.5.

Also as far as installation goes is this going to be a lot harder of an install because of the heat and ducting compared to a non ducted unit? And can I still use my current thermostat or do I need to upgrade that also. Here is what my current thermostat look like..