Maui camping advice

Hi there,

I'm thinking about camping in Maui during my visit at the end of April
and early May. I'll do one night at Hosmer Grove at Haleakala NP, and
my first question is, is it easy to secure one spot at this
campground? It sounds like a good place to stay overnight and get up
early for sunrise, so could it be too crowded to get one spot? I plan
to hike the Sliding Sands trail that day and do not want to return
there only discovering there's no spot left. Another question is, I
heard it could be pretty cold up there, so what kind of sleeping bag
should I bring? 40deg bags, or more like 25deg bags? I'll do another
night at Kipahulu near Oheo Gulch, again is it easy to get a spot
there?

Thanks for looking and any advice would be greatly appreciated,
Ryan