The OP never told us how old her trailer is. I will assume it is new enough to have an automatic switch over valve. If so, fill both cylinders and open them BOTH up slowly. Notice which direction the valve is turned. WATCH for the red to show up in the valve and when it does, switch the valve to the other direction, turn off the valve to the empty cylinder and remove it. Refill it and put it back on the trailer, hook up the hose and slowly open the valve.
Now, if you don't want to check for the red indicator you should turn only one tank on and wait for this to run out. As posted, this will happen usually in the dead of night, during a cold and often rainy night as it turns out. Get out of bed, get dressed, get a raincoat on and grab a flashlight and go out and switch the valve over to the full tank. Enjoy the black cold and rainy night while you are out there.
In my state I pay by the gallons I have used. I just traveled into the middle of the country and took an empty cylinder to be filled while is was in Wisconsin. The guy put it on a scale and weighed, told me it was empty and filled it. After weighing it he charged me for 7 gallons. After asking him, he told me they don't measure by the gallon but when I was watching him run the pump I noticed a second system that did measure by the gallon just like we have here at home. He told me that larger motor homes and some campers can't remove their cylinders so they measure the gallons in those situations. What? I asked him the obvious question and he told me doing it by the gallon cost the customer more in the long run. Never heard of such nonsense.
2017 Toyota Tundra, Double Cab, 5.7L V8
2006 Airstream 25 FB SE
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