You can tell if your split charge relay has kicked in by measuring the battery voltages at the terminals with the engine running. The leisure and main batteries should be the same and somewhere between 13.8 and 14.4 volts. If the LB voltage is lower, the relay hasn't energised. Try it with the fridge on and off, and see if giving some revs helps if the relay hasn't engaged. (Be careful measuring - moving parts, etc...) It could also be the wires feeding the propex are a bit on the thin side and you're getting too much voltage drop. When it's cold (ie when you need the heater) the battery voltage also drops slightly, these two combined effects could cause the propex to register an under-voltage and refuse to start up.
What he said, switch the fridge off on your way home is going to help. Initially the battery will take all the amps it can get.
