I can't help with a halfway spot, but I can give you 2 great boondocking sites near Bryce. I have found the Forest Service offices to be very helpful if you will stop in and ask about camping sites. They have great maps too.
Ruby's campground just outside Bryce Canyon has a dumpsite for $5 that has good tasting water.
Barb (RVM18), Sena (capuchin monkey) & Lily (Maltese/Yorkie) FMCA#F466348
"Homer" Ď11 Shasta Cynara, pulling "Smarty" '12 Smart Car toad
The Journey is Our Destination. Full-timing since 5/18/2011 - with no end in sight