As others have said open a hot water tap, better yet on all hot water taps several times each. This releases the air in the system and allows water into the tank and pipes. I drain my HWH, Fresh water tank , & low point drainsif I'm going home (I store it off site). I just don't want it sitting around for a couple of weeks with any water in it. I installed a short hose with a drain valve on the end of it to facilitate the draing of the HWH so I don't have to mess around with that plug, has worked great for years.
F250-7.3 L, 2012 305SK--Happy Wife, Good Life