Your engine coolant line exchanges heat for the hot water heater at (say) 180-190 degrees fahernheit. Eventually the hot water in the heater gets to that temperature. You obviously can get scalded by that temperature water.

If you valve doesn't deliver ANY cold water, it's defective. When you turn it COLD only, does water come out? Then it's just a question of finding the right mixing point.

When you have "shore power" generated hot water, it should be considerably less "HOT".