First thing I thought of was a bad ground. But if the brake light works, that's not the issue. Do you have a multimeter, if so it would help to locate the issue.
I would start by making sure you have a good contact at the 7pin connector. Might have corrosion or debris in it. If you have no power to the connector check your tv fuses. I found out the expensive way, that you have a separate fuses for the 7 pin connector. On my tv, they are under the hood, in the aux fuse box. If you have power to the 7 pin connector, follow the tt wire harness, depending on you tt it will go ether to a metal jbox on the frame or go through the floor of the tt. At that point check each wire nut. You may have a loose wire connection.
I'm not good at posting images from my phone. You can Google 7 pin wire harness, to see what color wire and which spot is thefor the turn signal in question.