Long ago I had a stubborn boar who used to eat only pellets, some grass and carrots. Nothing else!
Also my sows now eat very few foods, but at least they eat the right ones... the bell pepper for example...
About fruit, no, they don't have to eat fruit and must keep themselves far from any font of sugars, therefore it is not a problem if they are refusing an apple... as they should not eat it.
Try to give them fresh grass, the only food which contains all the minerals, vitamins and fibres they need. I give my sows only a very little amount of vegs (and one teaspoon of pellets, and not even daily), but they eat a huge amount of fresh grass.
Anyway, I use also a supplement of vit C in drops, the one for children. Maybe it is only wasted money, but I prefer doing like that. The supplement is very little, though.