A decent campground on Hwy 89 just north of the junction to Zion is Bauer's Canyon Ranch RV Park in Glendale, Utah. Stayed there a couple of nights in March. Has good gravel pads, long enough for your rig & toad, grass between sites. Pretty quiet, even with hwy just a short block away.
Sites are somewhat narrow, and all utilities are at the same end, so you'll get to know your neighbors!!! Nice laundry.
Town doesn't have any services, but aboot 10 miles south is groceries, hardware, and fuel.
Not really between Zion and Bryce, but not too bad of a drive to either park.