How can I fix my Dometic 7291 L?

We have a dometic fridge in our recently purchased motorhome. It is working intermittently. Sometimes it will work on both gas and 240v all day. It then just switches itself off (can be on either/or power source) and wont start again. We could go back to it in a couple of days and the same thing, looks like it works again for a while then switches itself off. It has behaved like this on both gas and 240V

Any suggestions