You will also have to go into the receivers settings and change the setup to SHOW SD/Hide HD channels as, by default, the receiver is setup to NOT show any SD channels if HD is available.
A BBC is not required to receive SD programming. The message you are seeing about needing one indicates the settings above are not correct. You can also install a Dual LNB in your 18" antenna. It looks exactly the same but has 2 (dual) outputs allowing to you connect both tuners. You could also add a SWM8 kit with PI29 if you connect a dual LNB and only need 1 coax running into the coach to power both tuners.