I can be wrong, but the leaves look a lot like improper fertilization/fertilizer burn. You said the plant is about 15 year old. Has the plant been repotted in recent years? It looks really small for a japonica at that age.

As long as it is still alive, it is quite possible to bring it back to health. It will take 2 or 3 years, if not more, but it can be done. If it is being fertilized at a regular schedule, I would stop any fertilization. If it has not been repotted in recent years, I would repot it in a fresh new mix in fall and give it a slightly bigger container.

Good luck.

The camellia has looked about the same for many years.

The root ball is small so the container-size is probably OK.

In January 2017, I repotted the camellia in E. B. Stone's "Azalea, Camellia and Gardenia Planting Mix" incorporated with plus E. B. Stone's "Azalea, Camellia, Gardenia Food 5-5-3"

Japonica leaves cannot handle strong sun well. In my zone, 8a, the japonica leaves brown out similarly and mine get sun for just a few hours but those are strong sun hours; only sasanqua leaves can handle the sun. I would try limiting the sun to morning sun until 11am or so and no afternoon/evening sun.