RKDave,
Yes, that is the route.
I would fuel up in Flagstaff, then it's about 200 miles to Fredonia. There is a Chevron station at Jacob Lake, but I don't remember if they have diesel. (Brain fade, I think they do)
Even if they don't you should easily make it to Fredonia. You can stop at Judd's Chevron in Fredonia, or drive 7 miles north to Kanab.
We usually have better diesel prices here, as there are 7 stations in Kanab. Plus there are many choices to stop, take a break and eat in Kanab.
PM me with any questions. I drive the Southern Utah and AZ Strip areas everyday. Travel safe and enjoy the scenery. I love driving all these routes out here and there are absolutely no bad views.
Where are you staying at Zion? Trace