Zion Ponderosa right outside the East entrance to Zion national park does the horses as well as ATVs, guided canyon hikes, paintball, fishing, shooting, rock climbing, etc. Not exactly a dude ranch but similar and a great experience. In addition to doing the activities onsite the wife and I took a day and went over to Bryce, hiked around and then did another horse ride with a company near the Bryce Canyon Country Rodeo.
They've got a small full hookup RV park onsite. When we stayed the sites were more expensive but it was all-inclusive, it looks like they've gone to an ala carte system with each activity being a separate fee. To my eyes it's not as good of a value but you can look at their site and make your own judgement.