Add a power source to a fridge van

So basically I have a fridge van. But I want to add the feature of adding a power source. So this will allow me to plug the van in when the engine is turned off to keep the food in the van cold. I’m wondering is this a possible job? And how to do it

Extension cord from the fridge to the plug?

But it doesn’t have the feature to plug it in. So what do i do?

I think we need a better explanation of what you have.