Tiffin had Freightliner put the plug under the dash behind the instruments. Unfortunately, they did not leave the plug empty. They used at least the power and ground lines for other behind the dash functions. I have the cable colors, and plug configuration, but it is in a .PDF file and I don't know how to attach it in this forum. If you PM me with your email address, I'd be glad to send it to you. Once you locate the plug, you can splice your brake controller into the wires without a problem.
The easiest way to find and access the plug is to remove the screws holding the dash instrument backing in the dash and pull it forward. Beats standing on your head.
2013 Tiffin Allegro Bus 43QGP
2011 Chevy Silverado 1500 Toad