I'm not sure how your HOT WATER BYPASS is configured. My trailer is setup using three BYPASS valves and looks this ..
AS you can see by looking at the bottom picture you can have water coming out of both COLD and HOT water lines and still not have water inside the hot water tank.. What you don't want to happen is to fire up the HOT WATER HEATER "without" water inside it it. It will burn out your hot water element if there is NO WATER in it...
Hope this helps understand what was said above...
just my thoughts
Roy Ken