There is no right or wrong answer, only what suits you. Possible problems with agency work are that no hours are guaranteed and you can be inexplicably left out of the plan at any time, without really getting a satisfactory explanation why.
While continuing at RM on decent rates (for quite easy work) might seem like a good option, I would say that getting some experience and getting a full-time position somewhere is a more secure option. To do this on class 1, getting experience may involve biting the bullet and driving a bigger vehicle for less money. This is where having a 1-2 year plan may pay off, but may require a bit of patience and perseverance.