There is no inexpensive EASY way to install/add a AC to connect to the Furnace floor ducts. RV's are not designed to have that done. There is no room AT the existing furnace area and that area is where you would have to have the forced air start at. Your best bet, is to either replace your roof AC with a larger unit or ADD an extra Roof AC and have 2 if you need that much cooling. The reason local HOME AC companies cannot do that job is because there is NO AC unit designed to do that. They are also not qualified and understand how RV's are put together to do what you want done. Doug