Drawing DC current from batteries on charge controller
As you are swinging the batteries between 56v and 42v, they are already getting a very hard life, especially the 42v end. I am assuming they are lead acid, and I am hoping they are deep cycle types. I doubt that the controller is doing any coulomb counting, there's no real need to do that with lead acid, so putting an extra load on the batteries is unlikely to worsen their cycle life, except to the obvious extent that more load possibly means more cycles. Beware that more load does not mean lower minimum voltage.

If you want longer cycle life, then limiting the bottom voltage to 44v or even 46v would be well worthwhile, if possible. You would win financially by limiting the low voltage, and putting an extra battery string in parallel with the existing to make up for the capacity being reduced by limiting the voltage swing. The better cycle life would more than make up for the higher investment in batteries.