Since the OP didn't have a problem last year and has been RVing for several years, we have to assume there is a problem with the water heater or plumbing.
The photo he recently posted shows the valve between the cold and hot to be closed, which is the way it should be. But, somehow the cold water is mixing with the hot. One possibility already mentioned is of the outside plumbing faucet valves left on and the water turned off at the hose end. Another possibility is one of the plumbing connections under the kitchen or bathroom sink is turned allowing the cold and hot water to mix. (My old HR Imperial had that option under the bathroom sink. It was to allow winterizing the coach.)
We've never run out of hot water and my wife enjoys NOT taking a Navy shower. (I do, but she doesn't, so she showers last. But, she doesn't run out of water so we're both happy.) So his problem must be in the plumbing someplace.