High Top Club Joker - Fridge problem

[Glenhyrst]

The problem is that the fridge will not reduce its temperature to below 8.5degC even on the highest setting. The thermometers used have both recorded 5degC in our domestic fridge.
It's an Isotherm Model CR4 9E2. I can hear the compressor run and switch on and off. Door seals seem fine.

The van has 2 months warranty remaining and I guess I will have to return it to Wandahome and leave it with them. Past experience suggests a fix will not be high on their job list and I would avoid returning to them if I could.
Is anyone aware of whether there is anything I can do myself to find and fix the problem? I suspect not, but worth a try. Thanks.