I have recently discovered a problem with my roof top air conditioner. 
The unit would start up and blow cold air for a couple of minutes then start blowing warm air Although the compressor would continue to run.

Having had a little bit of experience with commercial refrigeration I decided that the problem was most likely condenser. To test my theory I climbed on top to see if there was any air movement around the vents leading toand away from the rear of the unit, there wasnt, so I then made the decision to remove the unit from the van as there was no other way to gain access to the screws that secure the top cover. 
With the Truma, these screws are only accessible from underneath the unit. Having said that, the problem that I am speaking of, could happen to any brand of air conditioner. I have even had it happen to house hold air cons.

Once I had the unit on a couple of saw horses and removed the top cover I wasnt at all that surprised to find that mud wasps had built several nests on the condenser fan and prevented it from turning.

I have now installed fly wire over all vents on the outside of the unit and while I am aware that this may reduce some of the air flow to the condenser it is better than no air flow at all.

The unit appears to be operating successfully at present.