Here is what I have for watering trees. It uses the same pump to load and unload and a spray boom could be used but I belive a seperate pump would be needed to spray.
Attached is a photo of my water tank. It is a $40 300 gallon container that was used for a food additive. Poly plastic with galvanized box, plastic pallet, and plastic 2" ball valve. With a 2" valve you could easily gravity flow to empty. A 2" PVC header with 1/2" holes would empty pretty quick. Eliminating a pump would cut costs a lot and allow you to put more into the trailer. I tried to set up a hose with rope such that when I pull the rope the end of the hose is above the tank top = no flow. When I let out the rope the hose drops below the valve = water flow. Cheap valve. It did not work well but was an implementation problem, not an idea problem.
Anyhow, if you are making a trailer you could move 300 gallons at a time. A small tractor could pull the trailer. I use my container on a 3pt forklift. It is 2400 lbs full so I know it is back there. I water oak trees.
You could use a 2" water pump to fill the tank in about 3 minutes. You can leave the pump at the water hole and not have to pull it with tank.
You can clean a sprayer tank with amonia and not worry about contaminating your water with roundup.
Email me if you want a pic of the hose valve. It was made to water trees. Turns on/off the water without getting off the tractor.