While I think that broadly it is true that "timeless" is a marketing term, there are works that do in fact resonate no matter how far removed from their original context: The Epic of Gilgamesh, The Odyssey and The Illiad, Beowulf, Le Morte d'Arthur, Dracula, The Lord of the Rings. "Timeless" stories exist, for sure.
Not that I think any D&D novel is going to be one of those. Absolutely aim that thing at the contemporary audience.
I think alot of FR novels are timeless, most in fact. It helps its not set on Earth.