They corrode and leak inside, real common problem with some water valves. Especially older ones. Take a clamp and clamp off one of your hoses and see if there is any difference. Does not matter which one, just stop the flow of hot water in or out of the heater box. See if it's any cooler inside while your driving. I'm betting it's leaking internally.

I have some vehicles in my work fleet that I clamp off hoses in the spring and take off the clamps in the fall. Cheaper that replacing water valves that don't last long enough. They don't leak externally but do internally.

I use 2 pieces of 1/2" aluminum angle that I put 2 1/4-20 bolts through to hold them together. It don't hurt the rubber hose.
The picture above:
Those hoses above looked like they were exposed to oil? That's what causes them to expand like that and I would replace them soon.

Edit:Picture, this clamp can do both hoses.