Orbit 180 FQ New owner questions

A keyed battery switch should be close to the battery on the tongue, maybe attached to the frame, underneath. I would assume the mfg's instructions to turn that switch off is so the tow vehicle's battery is not drained while on the ball.

I don't have those black tank indicators, I shine a flashlight down the toilet. I read a lot of reports that the indicators are not very accurate. What kind of a seller would leave a half tank of waste in the tank? I imagine it could happen.