ND, that's not the problem on my truck.

I took that valve off 3-4 years ago and replaced it with a manual valve.
Both hoses get HOTTTTT!!!!, but the air coming out of the heater is only lukewarm, at best, once the outside temperature gets below 45 degrees or so. I've checked the temp of the hoses with an infrared thermometer, and they are both at 185+/- outside of the cab and inside of the cab.

I've tried reversing the hoses, with the same results.

I've also had my heater core checked, and it's clean as a brand new one inside.