Not enough info to give too specific an answer, need your van/MH details for anyone's knowledge of specific vehicle to help you.
In my caravan, the mains sockets are on two different MCB(miniature circuit breaker)/circuits, could be the same in your van and that one MCB has tripped. Where you would find the MCBs on your van, to check whether tripped or not, depends on make/model/year.
Completing the 'Outfit Details' in profile or stating in post is essential to get the kind of answers you need.
If it's not the circuit breaker, look at the rear of the ones that aren't working.
Often caravan sockets are wired in series, (like a string of fairy lights) so the wire loops in and out until the last one has only one cable going in. I'm presuming that single one is one of those that isn't working, if that's the case, and track the supply cable back to the next one that IS working and have a careful look at that socket, there may be a loose or burnt wire that should connect to the other two.
Needless to say, all this without any mains power connected. Good luck.
Dave.
------------- Give a man a fish and he will eat for a day,
Teach a man to fish and you can get rid of him for a whole weekend.