Hi there. Your approach is one way to do it but you're talking about 10 times the amount of antifreeze you'd actually need and while it doesn't hurt to put it in your FW tank, it's an annoying start to the next season with foamy water that is harmless but tastes and feels weird until it's all flushed out.
There's a procedure on your manual that should help, there's one in the Guide for Newbies that should have come with your trailer and a bunch of threads in these forums that will help you figure it out.
If you haven't yet, get the Camco winterization kit. It has a valve fitting you put on your water pump that allows you to attach a hose to jugs of antifreeze. Your pump pulls the antifreeze in from those jugs, fills the water lines and pushes out any water that could freeze. Make sure your water heater bypass is in the right position (ask me how I know 😕