New AC
I can tell you the price difference you quoted on the fan motors has nothing to do with the unit being for an RV. It seems like the pricing on those small motors is totally random, but I was told it has to do with economics of scale. If they have to set up the assembly line for just a few units, they have to charge more that if they make thousands of them.
That said, there are RV split-system A/C units, but they are bound to be expensive and require professioinal installation. As Gary said, your best bet is to stay with the original style. A different style of A/C unit may be cheaper in the crate, but the modifications you'll need to do to get it installed and functional will more than offset any upfront savings (kind of like tryng to put a Ford engine in a Chevy).
Any A/C unit that is not installed properly WILL severely disappoint you. You need to consider not only cooling capacity and power supply, but also air flow and distribution to have the unit be effective. These were already thought of with the rooftop unit you have now.
Just some random thoughts from a recovering A/C technician.