Our M/H is 31' and has a single ducted A/C unit. It was replaced last year with a larger unit on the roof. I can't remember the BTU's sorry.
I live in Georgia so you can imagine how muggy and hot it is here in the summer. That one A/C unit cools the entire coach to 72 Degrees without any problem. The Bedroom stays just as cool as the front area. There are 2 ceiling ducts in the BR and I think there are 8 in the rest of the coach. My only complaint is the bathroom (closet)isn't ducted. My DW just keeps the door open a few inches and it keeps it cool enough to tolerate.
We wanted a m/h without slides. That may be the reason why our 1 A/C unit works for us. We aren't cooling the extra space. With all the slides they offer I can see the need for 2 units.