Hi,
Without seing the trailer I wouldn't really be able to comment, I have no experience with them, but I don't see why you cant use twin and earth in the trailer so long as its all in trunking or not going to move etc, you would only really need to use flex if its being moved around inside, which i would imagine its in trunking. I don't know but there may even be something in BS7671 about it, ill take a look.
As for the PAT testing, this would only be for the appliances, fixed or stationary, so pretty much everything in there at a guess.
The inlet to the van, it should be a plug, not a socket, so yes it should be male else the lead you use to connect it will be a male and have live exposed pins which is very dangerous. This is easy enough to change, you would just need to buy a couple of new ends thats all.
Did he say why it needs rewiring with flex?
Some photos would be good if possible please.