Fridge problems

We own a 2010 Lunar Ariva which we bought last September. We have now be 'out' seven times and we seem to have a problem with our fridge. It works perfectly when we use gas; it starts quickly; it cools quickly and stays cold. However, when we use mains electric, it doesn't cool quickly or at all. We now start the fridge on gas and once cooled, switch to mains electric. Even when we do that the fridge warms up over time. I have read the manuals; I think we have done all the right things but I think we have a problem. 

The work should be done under warranty but our supplier says there is no problem and when they test the fridge on mains electric, it works according to them, properly. On site however it does not. Am I missing something? Is there some 'secret' switch? Has anyone else had a similar problem? Advice/help would be appreciated.

I'm taking our van in AGAIN in a couple of weeks for our local workshop to have another look