Definitely a no-no, unless you don't mind dealing with roof leaks and other damage later. If your 5er came factory equipped with a roof ladder, it is re-enforced enough to support your weight for maintenance, but definitely not designed for entertaining a bunch of folks.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.