Does your water heater have an electric element? Locate the make and model? It is normally on the outside water heater access panel.
You should not be operating the 120 volt water heater element and AC unit at the same time.
Look at your 120 volt circuit breakers. They should be marked as to what amperage.
Disconnect from shore power, remove cover to the 120 volt circuit breakers. Look in the back of the panel for the buss works that has the white (neutral) wires connected. Remove them one at a time, look for any signs of melted insulation or burnt wires clean the copper apply a coating of electrical grease, available at most hardware stores, re connect each wire.
Inspect each connection on the circuit breakers. clean and coat each wire with electrical grease.
Look closely at the male cord body on the shore power cord. Look for any signs of burnt connector pins or insulation.