Lack of power on hills while towing

[Busy B]

Hope you guys can help me diagnose a problem with my '08 6.4l. When towing uphill with my 5th wheel the engine will start making what can be best described as a loud howl which has a lower tone than normal turbo whine. At that point I lose a lot of power. Hills with any significant incline I usually can't go over 45 without this happening. Any tips for fixing this problem would be greatly appreciated!