The spigots and caps on the aquatainer's have an NPT thread. Long ago I screwed a plastic hose barb with matching NPT thread and left it permanently connected to an extra cap that I had. I use the cap with a piece of tubing connected to it when I sanitize. I have different size aquatainers that all used the same size threaded caps. When I sanitize I use my 2-1/2 gallon aquatainer with the cap and hose to add 1 cup of bleach and 2 gallons of water to the FW tank, then top off the tank with water.
I can also use that cap and hose on a 7 gallon container to add water to the FW tank or in freezing weather I can bring the 7 gallon tank inside and connect it to the winterizing hose using the hose barb on the cap. Then I can lay the container on its side. I can then open the vent cap when I 1st start running the pump. After enough water has run from the container I can leave the vent cap open.
I have used the trailer in cold enough weather the lightly insulated FW tank intake started freezing up. I was able to get by for a couple of days using a 7 gallon container. Then it warmed up enough the FW tank started flowing again.