>When I tried setting td to use width: 100% it messed up everything. So I removed td changes and changed fullwidth class to use 100% (which I tried originally but haven't tested properly). This time it worked and I think I like it much better to fill all the space.

The use of 100% fits well what the master parent has a fixed width. Otherwise, you can end up with very large content. I'm surprised however that it worked without td at 100%. Might be interesting to see an image.