[folivier]

Smagown has added 3 roof a/c's to his coach. You might PM him and discuss it with him. I saw his coach and he did a nice job. The scary part is cutting a 14" square hole in your roof. Once you get over that then it's just a matter of running the wires, you may be able to use the wires from the basement unit that doesn't work. I think Newell charges $4000 to add a roof unit. Might be able to get the seller to discount the price to cover that. Another temporary option is to buy a portable (roll-around type) a/c and just plug that in with the exhaust running out of the window. Of course that takes up valuable floor space.
Good luck.