The 5 pin is flat design, and the 7 pin round right? I have a 7 round to 4 flat converter that came with my Duramax. I use it with my Tundra as well. I havent chased down the 5 th pin, just have used the 5 pin flat on the trailer and plug it into the 4 pin flat of the converter. Everything works fine. Not sure if the 5th pin is for live power, or perhaps elec brake. Any trailer shop or carquest or Napa will have 7 round to 4 flat converter.
ryan