I wish there was an easy way. Often enough, I often need to empty my 20G ATO kalkwasser tank, sump, QT tank, etc for cleaning etc ... The last 1/2" is always most labor intensive with a sponge. Shop vac is too big of the deal to me. Ideally there is a dimple or low spot at the bottom of the tank to allow the last drop to be sucked out by siphon or pump (hard to find pump that allow hose to connect to the intake side too!), or a drain valve at the bottom (but then you have to jack up the tank).
Plumbing could be a science if you want it done right for minimal maintenance work.