If the pumps run fine with the van's engine running, then it sounds like a discharged battery or one that hasn't got capacity any longer.
If you are relying on your split charge relay to fully charge your batteries then you may not being doing the mileage to achieve that.
I'm not sure of your exact setup. If you have 2 leisure batteries, each supplying a control and pump, then I'm confused how one can influence the other.
I run 2 pumps and 2 controllers from a single 105amp leisure battery without issue. My setup also has a split charge relay but I don't do many miles so need to supplementary charge the battery every few days to be sure.
If I was running 2 leisure batteries, each with a pump and controller, then I would want to run each unit seperately and have a split charge relay for each battery.