My suburan hot water in my 5th wheel camper only makes warm water, never hot. what is the problem?

First, let water heater heat up until it shuts itself off. Now carefully, ( it should be hot) open pressure relief valve slightly and catch some hot water in a cup or container. Compare it to what comes out your tap, that you say is warm only. If that water from pressure relief feels hotter, then check your by-pass valves for proper operation and position on rear of water heater, as it may be in a position that is allowing cold to mix with hot, creating warm. Cold line will go direct in the bottom fitting and hot will come out top fitting, and make sure any valves in between those 2 lines are in off position. If all is well there, or water from pressure relief is only warm as tap as well, then buy a new thermostat and ECO for that particular model water heater from RV dealer/service center, and replace them. They are behind rubber cover on upper left side on water heater. When cover is off, they are held against tank by 3/8 nut. When you replace it, snug nut up good but be careful not to go too much and snap that small brass stud that nut goes on. Pay attention to position of old one, so new one goes same way.