While I have not stayed at the Zion River Resort there are several fine campgrounds in and on the way into Springdale, I have always stayed in the park but they do not have full hookups only elec in a couple loops. Ruby's is fine as well.
As far as length of time it depends on what you want to do. I have hiked virtually every mile of trail in Bryce in few days. Otherwise a couple days in Bryce is probably more than plenty. Surrounding area could add more time for sure again depending on what you like to do and how far you are willing to day trip. Zion I have not hiked all it's trails but it is a much larger park. If not hiking a couple days in Zion would be plenty to do the pullouts and view points without rushing and some short hikes as well. Probably another day if you went up into Kolob Canyon area of the park.
