Simplicity......I think OTR is designed as it is because for a commercial coach to be practical it needs fast turn around in service work. The OTR as built is centrally located and the condensing side, evaporator side and the compressor are all accessible by opening a door.

For heating in my Liberty I can use the OTR system which is a whole coach system with one set of temperature control, or the Webasto which has multiple thermostatic controls. When the coolant is heated the Webasto burner never fires, but the heated coolant from the engine is circulated through the various heat exchangers throughout the coach.

As to using water for condenser coolant don't we still have the problem of getting rid of the heat the water has absorbed? Using water cooled CA condensers would be easy because they are located in two general areas so plumbing would not be excessively complex, but the heat still has to be removed from the water.