Another vote for Bryce while going from Moab to Zion. An alternative would be a day trip from Zion area to Cedar Breaks NM to get just a little taste of what Bryce is like.
Also a 2-hour stop at Goblin Valley State Park is worth considering for the totally bizarre sandstone structures.
Zion River Resort is very nice, but pricey. The main advantages are being able to make reservations as far in advance as you'd like, and the pool, etc. Watchman CG in Zion is a better option if you're not going to be spending a lot of time at the RV park. I'd start with reservations for Zion River Resort and then cancel them if you can get a space in Watchman.
If staying in/around Springdale (outside the West entrance to Zion), be aware they don't allow restaurants which have the same name & menus as other locations. So do some reading up in advance on where to target if dining out. The exception is Subway, who recently won a years-long suit over the issue. I find Oscars particularly fun, charming, and filling.
2013 Heartland North Trail 22 FBS Caliber Edition
2013 Ford Expedition EL with Tow Package