As already mentioned, ample sunlight and pruning are critical to mantaining fruit production in wild apple trees.
Before pruning, trees shading the apple tree should be cut down. Apple trees that have been overtopped for a long time by taller trees should be released by removing the shading trees and only the dead, dying and diseased wood should be removed during pruning at that time. Pruning should be completed sometime later before early spring growth. Releasing a tree and severely pruning it during the same year could throw it out of production or kill it.
Pruning neglected old apple trees for wildlife is much different from pruning orchard trees. You don't want to change the shape of the wild tree or top it. Orchard trees need to have their tops pruned severely to facilitate easy apple picking but that's not important when pruning wildlife trees.
You can fertilize the trees too but this often isn't necessary if you have decent soil. I have rejuvenated many old apple trees and it's rewarding to go back in a year or so and see the apples hanging!