I have visited the south rim of the canyon twice. Once the first week of November, the other on the last few days of Feb/ first few days of March.
Both were before we were RVers. However the weather was great for RVs in Flagstaff and the South Rim both times. In mid-April 2016 out best RV friends were at the South Rim perfect weather no snow. In late May 2012 they were there and could not get the Class A up the highway due to snow.
1. If you have never been there, go. It is a fantastic place.
2. April SHOULD be okay.
3. If you want a guarantee on weather, visit a chapel because only he can give that.