Also, unless you 'crack' one of the faucets when opening the low point drains, not much might run out. Not opening a faucet creates a vacuum in the plumbing, keeping water in there.
The water from the low point drains comes from the entire plumbing of your rig.
Gravity allows the water to drain out when they are open.
Depending on where the water heater is located in relation to the plumbing, sometimes it will drain the water heater really well, sometimes not. That's one of the reasons it's always advised to remove the drain plug/anode rod on the water heater itself when draining.
Also if the water heater has a built in check valve in the hot outlet, it may not drain well from the low point drain either.
2016 Flagstaff 8529IKBS Diamond Package 5th Wheel
Ford F-Series SCREW 4x4
TST Tire Monitor - Sinemate 3500w Gen.
2016 Nights Camped = 46