Sleep in your car. If you're cool about it, the Pinkham Notch staff usually look the other way as long as you're quiet and not being a nuisance. (E.g. best bet is to roll in anytime after 8 PM, shut the car off and go to sleep).
If they kick you out (which they've done to me before, first time was because I was with a large group and a few of them were being obnoxious and loud, the second was because it was the middle of the summer and I don't think they like people sleeping in their car's then) take a right out of the PN parking lot and drive about 100 yards down the road - there's an overflow lot where the never check. I've seen people pitch tents there before...