I never tried to make ice in the refrig on my 2000 H340 but mine will keep a bag of ice frozen for the whole summer at a middle thermostat setting of # 5.
Some suggestions: Check the fit, seal and locking of the lid. Check that you are not loosing cold air out of the drain hole in the bottom of the refrig. Every 2-3 years remove the forward cowl from the compressor unit and vacuun the dust and dirt from the coils. Also make sure that both of the fans on the compressor unit are working. Lastly, you can insulate the coolant line that runs from the compressor, under the settee bench to the refrig.