despte an isolation switch being used when we are parked up
Is the isolator a manual switch or does it work automagically ("split charger", relay, diode isolator, dc-dc, etc)?
If the leisure and starter batteries really are isolated then it's a chassis problem. The classical approach is to measure current leaving the starter batteries and pull chassis fuses until the current stops -- that circuit is the problem.
occasional drain of the motor batteries
How long does it take for this to happen? A day? A month? If you have solar on the rig you could leverage it to keep the starter battery[s] charged. This would also work if you are on shore power while parked up.