Here's another way of looking at it;
If the cabover bed tilted on the front legs with a 250 pound man up there, then the rear legs have less than 250 pounds between them when the man is NOT in there. Put a scale under each rear leg and see what the weight is. I would bet that each rear leg has over 500 pounds per leg when on level ground and the camper level.
If the entire bottom of the camper is supported doesn't make any difference. YOu are still describing a teeter-totter with either the front legs or the front edge of the camper base.
Now if you REALLY wanted to test it, attach a ratchet strap to the back of the camper with an inch of slack. Climb into the over bed and if it does tilt, it'll only move an inch and you can climb back down.
But I would bet that you would need at least 1000 pounds on the over cap portion to lighten the back enough to maybe get it to tilt/tip.