I have noticed alot of people want to put a tent ontop.. well its been done for years and years on just about every kind of vehical around..So with that being said.. check out link below for ideas on how to make a tent for your XJ... just searching threw these pictures will give you loads of ideas and insite into what you need to do with out guessing so much.
http://images.google.com/images?hl=e...l=&oq=&start=0

 

for the price of one of those you could convert your AC to an air compressor, add the air tank and purchase a air mattress to lay in the back of your xj with enough money left over for "supplies".