Inko,
Bummer! I have to say I am surprised, 220lbs is not a lot of weight. I have a rack up there utilizing all three mount points, so I'll be a little more cognizant of how much weight I put up there!
If you like to build things maybe you can rig up a header across the top on the inside and load the floor with some vertical members. You probably could get some extruded aluminum I-beam, I wouldn't think it would have to be too big...
Anyway just thinking aloud, roof top tents are a pretty good idea, one I would consider, Let me know if you have any luck
thx
S.