Pretty sure it is standard on the 21FBS. I have a 24FBS and the black tank flush hookup is located on the rear wall of the TT. It's about eyeball height and should be labeled. Nearby may be your city water hookup also. Don't get them mixed up or you will have a mell of hess when you turn on the water.
Welcome to Jayco. You will find this forum a valuable tool for any questions you may have about your TT. This is quite the knowledge base with a lot of smart and practical campers to answer your questions. We have fun too.
2014 Jayflight 24FBS Elite
2009 F150 Supercab, 5.4L ,3.73, HD Tow Package, Equal-i-zer 1K,
Progressive EMS-LCHW30, TST 507RV TPMS, Hydro Life HL-200, Maxxis 8008.
Best friend wife and dog Cooper. RETIRED 6/30/16. WOOHOO! Now the "endless summer" begins.