You can easily calculate the RMSE errors and also some other errors like Mean Percentage error (MPE) by using the accuracy function.
ex:
If the name of your model is name
then RMSE is calculated as :
accuracy(name)
which will give:
ME RMSE MAE MPE MAPE MASE ACF1
Training set 0.05576569 8.313739 6.227064 395.7271 576.6584 0.5128541 -0.09583192