It all comes down to ohms law. If you have low voltage, you will draw more amps. More likely however one of those components has a short or partial short in it. I would bet on the water heater element. Best way to find out, is to find out what the normal resistance of that heater coil is and test yours. i am betting you will find it is too low and you have a partial short. I would leave it off and see if the problem continues. If it does not, you have your answer, replace the heater element. You obviously have to high an ampage breaker on that circuit as well, it should be tripping before stuff starts melting.
Also cheek the connections at the panel. A wire that is not tightened properly into the lug will cause what you are describing. RV's have vibration. Even in my house I check the connections once every couple of years in the panel. I had a fire once in my house panel due to a loose wire. In an RV its even more likely.
resistance = heat.