there's no benefit to not running in tow haul mode with that much weight, so I would definitely keep it in t/h, whether the land is flat or hilly, or all downhill or whatever.
Okay, I just got home from the Desert 100 in Odessa Washington. I am sore as hell.

I only road the FFR poker run but that was enough for me.
I was very surprised how the tow/haul setting worked. Way different then my last truck. (F-150) I am real happy with the way it worked. Odessa is in eastern Wa. and I live in Tacoma so I had a mountain pass to go over and several long grades. I can't believe how little I had to use the brakes. And the P3 Tekonsha brake controller I just installed for this trip worked AMAZING! No brake grab like I used to experience in other controllers I have used. Very smooth and even braking. Truly a very comfortable trip. My total gross was about 16k, both units weighed nearly the same, 8k. Not the best pics of the truck and trailer but I will get a better one soon. The next couple pics will be of the camping area and the race start. In order to get the whole camping area I had to take about 6 shots. To many to post but that gives you an idea how many people were there. The last pic is of the riders lining up for the race start. It was a running start, all bikes were parked and not running, the riders standing about 20 feet behind the bikes. When the cannon went off the riders had to run to their bikes, start them and take off.
Oh, the real last pic shows that it did get a little dusty from time to time.
Scott