There isn't much more I can add to Don's reply.
If you do a search on split charge relays under my name there is tons of information that is relevant to you.
We tried on 2 of the small vans (that now have 85 amph leisure batteries,) to run them off the van's starter battery. It didn't work for us as we don't do enough mileage to recharge them. Having to jump start a van half way through the day cost more than a leisure battery in down time.
If your pump draws 4 amps per hour and you have a fully charged 100 amp leisure battery, you theoretically have 25 hours of window cleaning available. If you work 4 hours a day actually pumping water, then you should again have 6 days of power available to you.
Unfortunately you leisure battery is best looked after by not being discharged to far. Ideally, recharging after it discharges to 75% capacity is best and not letting it go below 50% ever. Nothing destroys a battery's longevity than letting it run flat. We have also found that the battery works better if it isn't placed directly on the van's steel floor. Both the vans that have the batteries in the back have a thick piece of rubber conveyor belt under them to isolate them from the cold. The third van's leisure battery sits in a stainless steel roasting pan under the seat on a layer of carpeting.
A battery's performance drops off by up to 25% in very cold weather.
So even although we have scrs on each of the vans, my van especially gets a booster charge every second night. An intelligent charger can be left on all night.