Each of the national parks/monuments has an excellent web site for exploring things to do, camping, roads, special alerts, etc. Check them out. Here is Zion's and the information on the Hwy 9 tunnel road:
https://www.nps.gov/zion/frequently-asked-questions-about-the-zion-mt-carmel-highway-and-tunnel.htmIf you can't use the tunnel, a good way to go is from Bryce take 89 north a short distance past Panguitch to Hwy 20 taking you over to I-15, then south to Hwy 9 and come in from the west. Try to get an electric site in the park at Watchman campground. There's also a RV park in Springdale and further west toward Hurricane but in the park is best. You can easily get on the free shuttle. The shuttle also makes stops in Springdale.
Bryce - you'd fit in the dry campground in the park or else stay at Ruby's just outside the park entrance. The free shuttle stops there.
Capitol Reef - you would fit in the park's campground which is a lovely serene spot. There are also many RV parks in nearby Torrey.
You didn't say which highway you'll be coming in from but if it's I-70 get off at Green River on Hwy 24 going to Capitol Reef. Then take Hwy 12 to Bryce. Highway 12 is a gorgeous drive but has some steep areas. Not everyone likes it but we certainly do. It just depends on how you feel with your truck. Many, many RVers do it. If you don't want to, from Torrey/Capitol Reef continue on 24 to 62 to 89 and south to Hwy 12 then east a short distance to Bryce.
Have a great time. They're all favorites of ours!