So you're definitely at full capacity of your truck. If it were a lightweight trailer, I'd say turn the tow haul off when not pushing the engine. I would only turn it on when pulling my pop-up on hilly roads or driving in stop/go traffic.
Personally, I would recommend for you to keep the tow/haul on unless the road and wind conditions are favorable on flat land. You can sometimes turn it off to get that last gear for improved fuel economy in those favorable conditions. Otherwise, leave it on.
Now I drive an F350 diesel, tow heavy and I always leave it on. But it also will shift into all 6 gears and by leaving it on, I take advantage of the improved engine braking.
