Should I leave my Travel trailer plugged in or not?

I installed a battery cut-off switch on the positive terminal of the battery and hooked up a "battery minder" to it. So, that is the only thing I keep plugged in all the time. It keeps the battery fully charged, and desulphated to boot, and draws only about as much power as a night light.

When we first got our trailer, I left it plugged in all the time. It took three years of that to kill the battery. Evidently sulfated up.