If you hook it up in series with the hot water heater it would probably work out as Tom wrote. But if you T it off on its own loop in parallel with the water heater then airlock is more likely in that loop.

I have a Heatercraft I haven't installed yet, and would put it in series. Also pay attention to the direction of flow. I would want the cabin heater to be first, on the reasoning that it would take longer to heat the air if the engine coolant temperature is lowered by the water heater first, and taking the chill of the crew is usually a more immediate need than making hot water.