Keeping water hot

I have a Suburban 12 gallon heater that was suppose to be quick recovering and very hot. Ours was not. Sort of what you describe. I have a repair guy in Amarillo that is the best. Was talking to him, and he describes a couple of black round things on the front of the heater. Since I don't know exactly exactly what they are called, but he did. I think one was a thermostat. He sold me the part, and told me which one I needed to take off. I did as directed, and could not believe the difference. I had told the dealer when I bought the Cameo that the hot water heater was not as told it would be. So bad heat for about 3 years. One simple change and it works as described. Wish I could tell you what I changed, but I do know when you look at the heater it was the RH one, flat against heater.