I'm new here and wanted to chime in because I had the same question about poly over tung oil. From my research it is completely ok as long as the poly is oil based.
Here are a couple of answers from Rockler:
A. Michael Dresdner: "Yes, I would put a coat or two of oil-based polyurethane on for more durability, and it is fully compatible over the dried tung oil (or linseed oil, or any other drying oil for that matter.) In fact, you can apply it in much the same way as you did the oil -- scrub it on with fine Scotchbrite, then wipe it off. That will leave a thin coat that will maintain the look you want with no drips or brush marks. Add at least three coats, at one coat per day. If it starts to wear down the road a few years, add extra coats the same way."
A. Lee Grindinger: "You'll get there eventually if you keep applying the tung oil finish. Tung oil finish is varnish, highly thinned varnish, so your coats are very thin and you'll need very many. If you want a faster build use a regular varnish and brush or spray it on. There are still many varnish formulations that use tung oil. To keep the sheen low use satin or eggshell. Plan on rubbing the surface out with steel wool or rubbing compound to remove the nibs."
Hope that helps. I'm going for it.