OK. Have you drained and flushed the tank recently? Sludge can build up in the bottom of the tank and insulate the heating element from contact with the water. Do you have a propane option? If so, does the water get hot in that mode? If not, sludge would be a likely culprit.
Now, I guess it is possible that the heating element has shorted out and only a section of it is working. With SHORE POWER DISCONNECTED AND GENERATOR BREAKER OPEN, you can check the resistance on the electric element. Disconnect the terminals on the heating element when checking.
A good element should show a small amount of resistance (up to 20 Ohms) between the two terminals. And, a check of each disconnected terminal to ground should show an open circuit. An element not meeting those criteria is most likely bad and needs to be replaced.
There is also a thermostat to control water temperature, and it is possible that it is faulty. You could try adjusting the T-stat and see if anything changes. IME, the T-stat is rarely the problem.
Certainly, my first option would be to drain and flush the tank. Be sure the power is OFF when you do this or even a good element will be destroyed if it operates without being submerged.
TJ