had the same sort of problem on my old Class A, even though the water heater bypass valve was in the correct position to winterize---what worked on that rig was to leave the water heater "full" of water then flip the bypass & put antifreeze through the system-worked like a charm...afterwards open the drain plug on the water heater & drain the water out...best guess I had was that the pump would sense no back pressure from the water heater & would try to fill it (with antifreeze)--do my fiver the same way (in fact will be winterizing this weekend)