The OP did not say the breaker would not stay on but said 'it happened again'. I assumed he meant he reset the breaker and after some time it tripped again.
I don't know about the elements in RV tanks but I know for home water heaters you can buy different wattage elements. Is it possible the OP bought an element that was too high in wattage?
2016 Siverback 33IK
2015 Ford F-250 Lariat 6.7L V8 Diesel 4WD Super Cab
"If you don't know where you are going, any road will get you there."