RedDot has all kinds of add on ac/heater combos.

we are installing our 4th system on our floater trucks in the last year.  The factory ones just don't cool and with out the condensor up front the engine coolant actual stays cool.

Some of them may run half what that baby is worth though.  I would have to look at the slip, but I think the last one was $2500 for ac only.  which is alot cheaper than the first two, but after running them, I didn't care what they cost, just told them to order them!

I think it is www.reddot.com  Nope this is it http://www.rdac.com/Pages/product_pages/units_index.html

Just click onthe rooftop models.  none of ours are rooftop but I would think it would work for you.