I have been to Zion in March many times and although it can get cold at night, the days are usually nice, unless the wind is blowing hard...unhook your water hose at night when it is supposed to freeze and run off the inboard tank and you should be fine...
My favorite place is on the West side outside the Park and called Zion River Resort, a beautiful full service RV Resort, another advantage of being there before season you can drive into the park, during high season you have to take the buses, which works well, but when I am hauling around 100 pounds of photography gear it is a bit restrictive.
Enjoy Zion!
Jim