Its tough to get that last ounce or two out of the tank. You seldom need to get it completely dry, luckily. Some kind of hand pump or bilge pump should work for that problem. You may want to use a high-pressure power washer if you need to remove some kind of sticky residue. When pumping out...be sure to park the truck on the steepest part of your drive or a steep hill...to get the water to flow to where you can get at it.
In my case I used my sprayer with a Hypro diaphragm pump for many years with the agitation connected to the pressure regulator bypass hose. When not spraying, all flow from the pump goes to the agitation nozzles. I used a section of poly tubing to go from the top inside of the tank to carry the agitation flow to the bottom. A plastic pipe tee then allowed me to add four Delavan agitation nozzles. They were aimed to provide the maximum swirling and rotation of the spray solution.