Good news. The trailer is fine, wrong f-stop!
The drains, which are properly called low point drains, are to remove water from the water lines. To drain the lines you must first turn off the water supply, including the water pump. (Otherwise it is like trying to drain a hose with the water running!) Open your faucets to relieve trapped pressure in the system and to allow air to enter the line(s) as the water drains. Remove cap.
I think you are speaking of a water heater bypass system when you mention a bypass lever. If so, this causes the water supply to the water heater to bypass (go around instead of through) the water heater. This allows the water heater to be drained (separate procedure) so that the water system can be winterized (another separate procedure) without having to fill the water heater with 6-10 gallons of antifreeze. Note that the water in the hot water line remains under pressure as long as there is an active water supply.
I bypass, and kill all power to the water heater, before draining the lines or the heater.