Isotherm (Webasto/Indel) poor balance between fridge/freezer temps

[clydeps]

I have an Isotherm CR200 fridge/freezer that only has one thermostat. If I set it to get the fridge compartment right, the freezer is too warm (frozen stuff is soft.) If I turn it down further then fruit and veges in the fridge compartment freeze.

Does anyone have any ideas on how to fix this? It's not clear to me how the unit apportions cooling between the compartments (and the user manual is silent in this regard.)