Also, are you running it from warm? Many folk plug their vans in overnight before touring so that the fridge is already cold when the battery is asked to maintain temperature.
On the occasions we have to run from warm, the fridge does a reasonable job of cooling things down but it is in no way the temperature you would desire. Put some ice packs in it before starting out. That helps.
Randa

Have you checked the 12V relay on the BIP unit?

They are cheap and easy to replace. It looks like this and is plugged into the front of the unit.

Relay01.jpg - Bild entfernt (keine Rechte)

Relay is to right of the switches and is retained by a wire clip.

Relay04.jpg - Bild entfernt (keine Rechte)

BIP02.jpg - Bild entfernt (keine Rechte)

Basically, it's a switch. When you are hooked up to the car with the ignition on, you should hear a click as the coil energises and the contacts engage. The red light on the fridge should then come on and, provided the fridge 12V element is OK, it should start to heat up and begin cooling the fridge.

In some car/van combinations, the relay doesn't operate until the engine is revved slightly but on others, mine included, it energises immediately the car ignition is turned on.

You should get someone to start the engine and rev it while you stay in the van to check that all is operating correctly.

The contacts on the relay can stick and/or burn out due to arcing. With the old relays you could effect a temporary repair by cutting the outer case from its base and cleaning the contacts with an emery board. The newer relays are better sealed and don't come apart easily. In any case, they are so cheap (about £5) that it's simpler to change them for a new one. I now carry a spare.

Here's a link to the post I uploaded when I was having relay problems: Fridge Relay

Hope this helps.

Have a look here, Tony. Even cheaper than I thought.

Finder 12V relay