Hi Jason,
I've a merlin too, but a pressure of 80 psi, takes about 2 hrs for me to fill 250 ltrs. I guess the fill time is down to your pressure, You could up the production with a booster pump, though that could be quite an expense for one thats up to the job ( others maybe able to advise on something suitable) . Are you filtering to a tank in a van or into a static one? if latter how about fitting a float valve? or time how long it takes to fill and connect one of those garden tap timer switches.
Simon