Wabasco water heating system not circulating water

My friend has this monster size bus with a wabasco water heater. The fuel fired burner comes on and heats the water but the water solenoid doesn't open and the pump won't circulate the water. we have replaced the relay on the silver control box and sometimes we get 12 volts to the solenoid and pump. I say sometimes because sitting there with no request for hot water there is 12 volts. As soon as the burner comes on the 12 volts is turned off. Now, we jumped directly to the pump and solenoid (they are wired together) and they will both run. The fuses up front for this system are ok.