That might or might not be true - I'm not sure. On my Landie I close it off as the heater matrix is exposed in the cab.
However, I am very sure that all of the RV8's have a thermostat bypass circuit which is guaranteed to be open, and a separate parallel heater circuit which may or may not be open. The only thing that is worth being wary of is that if you shutoff a circuit that is normally open, then an air pocket may accumulate behind the valve, which will change in volume as the cooling system pressure varies, and make the volume in the header tank go up and down.
Chris.