Most of the boat ramps at Lake Mead are closed because of the ultra-low water levels. We were just there in April and stayed at two boondocking sites along the lake (Government Wash & Stewards Point). You can pretty much boondock anywhere you can access along the lake, but you do need to pay fees.
FEES-> Since Lake Mead is part of the Lake Mead Recreation Area there IS a fee here. It's $10 for 7 days, $30 for a year or you can get in with your America The Beautiful Pass. 15-day stay limit.
TEMPERATURE -> It is already getting HOT there. Right now it's around the 90's. Be careful of heat & wind in the area.
SPOTS -> Here's a link to the two spots we stayed at:
Stewards Point, Lake MeadGovernment Wash, Lake Mead