We're likely going to be hurting come Memorial day, don't expect the mountain to stay open longer.
As far as rent goes. A lot of people are trying to fill rooms for a month or so since people are starting to leave, check the forums on mammothmountain.com or mammothtimes.com classifieds.
In regards to camping, you can camp on BLM land for free for pretty much as long as you want. The nearest is about an 8 mile drive from town. I don't believe the Scenic Loop is BLM land, so if you did that, you could face a pretty hefty fine from the forrest service should they catch you camping at a non registered camp site. (I could be wrong about the BLM thing, but I'm pretty certain it's not).
The park is still alright at the moment, but by 1 it's pretty slushy. The upper mountain stays pretty nice though.
Then of course... it's now fishing season and the owens is lovely. So much to do this time of the year. Friday I was skiing, Saturday I was day drinking and partying on the sundeck in shorts watching the Grenade Games, yesterday I floated the owens river with around 30 people, tomorrow I'll be skiing. We don't have a mud season like a lot of other places.