Truck runs cold

[05PHATBOTTOM]

I recently installed Edge Insight on my truck. I was keeping an eye on my ect and eot. My temps wont go more than 158 deg ECT and 168 EOT. It is only 20 deg outside, but pulling a 8000 lbs trailer it should warm up higher than this shouldn't it. I even put cardboard in front of the radiator. Could it be a stuck thermostat?

thanks