I honestly don't know the answer to your problem.
But then I don't fully understand when you have the problem.
It sounds to me like each battery has its own charger. Do you leave the batteries in the van to charge them? What happens if you switch batteries around? In other words, you are trying to ascertain where this is a fault with the battery, or charger, or setup. (Problem solving is to do with when you don't have the problem, then when you do have the problem, and then working out what changed.)
Are both batteries leisure batteries?
I presume you have linked both these 85 amp hour batteries in parallel making it in theory a 12v 170 amph battery pack.
What do you do with the 'link' when you are charging both batteries if they are being charged in the van?
What happens if you remove this link altogether. Do you still have the same problem?
Tell us a little more about your Split Charge Relay. Is it an intelligent one and if so is it bi- directional. I have an M-Power Voltage Sensing Relay (same as Intelligent). It is the same as a Durite VSR. They are both bi-directional. To explain; a VSR isolates the van or starter battery from the leisure battery and only switches across once the starter battery has the starting current replenished. A bi-directional VSR will also work the other way round as well. If I plug my charger into the leisure battery it will take a while before the relay switches on and my van battery also gets topped up. But I have also noticed on occasion that the battery charger gets a bit confused with both charging together.
About four years ago I put a Numax 85 amph leisure battery in son in laws van. After a week of standing, the battery was only half charged. My son's van had the identical battery, just a couple of months older. This battery would stay fully charged all week if the van was standing.
First thing I did was to swap the batteries around, but now son's battery lost charge over the weekend. It wasn't the battery as son in laws battery stayed charged in son's van. So now we had a direction. Somewhere with SIL van there was a drain on his battery. It was sorted by fitting an isolator switch between the leisure battery and the Varistream.