sound like the tempering valve, they set them at factory to 45-50 deg i think thats legal limit at 50deg, we often get people asking for them to be adjusted upwards so they can wash dishes, but legally we can't go over 50. just be warned that if you do adjust it you may void your warranty for the water system on the van
if your looking for yours it's under the van close to the HWS, brass T shaped thingy with 3 pipes going into it.
hot-in, cold-in, and warm-out (the hot and warm should be red pipes) if it's gradually got colder it is probably faulty or blocked a bit no reason why you can't pull it out for a clean