Towing and transmission issue

[RGSMDNR]

I have an 07 f150, 5.4 supercrew...

I tow a 25' travel trailer that weighs about 5000lbs. My issue im having is maintaining highway speeds when the tranny heats up. Its fine around 150 or so... but anything above that the truck seems to run like a dog. It doesnt matter if im using the towing tune or running stock. To be honest i dont run the towing anymore because it doesnt have any top end. It seems to run better stock. Any thoughts on why the truck doesnt make any power when the tranny heats up?

thx