I have had the same problem. I have the Sargent as well and it is extremely comfortable for me, with the exception of the heat. I also have the Corbin, bought to help combat the heat issue. It is much more firm and covered in leather, so the heat issue isn't as bad, of course the shape of the seat leaves a bit to be desired. The seat is designed for those who weigh a bit more than me, to put it nicely. If you do not have extra padding, the seating position is a bit too far back, and the front hump is too pronounced. Still, I am learning to like it. My wife prefers the Corbin for its position for her.
I can do 800+ days on either the Sargent or Corbin. I did a 1200 mile day on the Sargent. If the Sargent was leather and a tad more firm, it would be great. If the Corbin didn't have the hump from hell, and had the good people at Corbin actually do what I asked, and what they said they would do when they made the seat, not make the seating position so far back, it would be perfect.
The Corbin is getting better, after 5,000 miles, but I wish they would have listened in the first place. Some have done a humpectomy with good results.
Overall the heat issue is cause by the foam conforming to your body a bit too closely preventing air from circulating. The leather helps, as does the firmer seat. I had a Sargent on my RC51 and I didn't have the issue, of course it was a very thin seat by comparison, so you couldn't sink in far.
Good luck. If you can have the Sargent recovered in leather it may help.