To avoid confusion over debits and credits, avoid thinking of them in the way that they are used in everyday language, which often refers to a credit as increasing an account and a debit as decreasing an account. For example, if our bank credits our checking account, money is added to it and the balance increases.
One way to reduce the frequency of this occurrence is to maintain an air pocket at the top of the water heater tank. This air pocket will form in the tank by design - however, it will be reduced over time by the everyday use of your water heater. To replenish this air pocket: Turn off the water heater.
There should not be any air bubble in your water heater, RV water heaters are not that much different than the ones in our homes. It the Temperature/Pressure relief valve is in an air pocket, it will not sense the temperature of the water. If it was over heating, this would be a steam pocket.