In addition to the above comments, I'd like to offer some further suggestions. All the current needed to supply the heater needs to pass through the thermostat mounted somewhere on the hot water tank. In my RV, it was conveniently mounted to the tank's exterior and accessible by moving a little insulation out of the way. The thermostat contains metallic contacts that open and close according to heating requirements. If they degrade over time, the current available for the heater could decline before they finally burn out. if replacement of the heating element doesn't help (as suggested by the above respondent), you could possibly look into this area, which brings me to another point.
I always carried electrical test equipment in my RV, as many other RVers do. I used a digital volt/ohmeter and a clamp-on ammeter for electrical testing. My meter was a professional Fluke brand, but for general usage an inexpensive one has sufficient accuracy. If you feel safe in testing electrical circuitry, or can learn, I feel such an item can come in handy in diagnosing an electrical problem (probably not important to have a clamp-on ammeter)
As far as testing water heater problems goes, if and when you replace the heater, take a resistance reading on the new element and record that reading. A good heater should read similarly. A burnt out one would be easy to spot. Obviously, a voltage check to the heater would be in order if you can safely do so, although I've seen instances where voltage may read correctly but not enough current was available.
In summation, procure some simple test equipment and learn how to use it if you feel you're confident in your ability.
