Definately incorporate a float valve. It'll be the best $15.00 you've spent. If your furnace is equipped with a humidifier, "T" into the supply line for that to supply the RO unit. As mentioned, use a ball valve there to disable the system for servicing. I'd avoid using the fittings which pierce pressurized lines...

You know I never though about going into the humidifier supply line, that is a heck of a great idea!!

A great idea it is...! A sign of wisdom, which comes with age, and enough screw-ups to write a book...!