The Roman Catholic Church claims that some corpses have supernaturally slow decomposition. Were those bodies examined by skeptics?
Yes, some of those bodies were examined by skeptics. Io and behold:
Sometimes the saints had been mummified by skilled preparators, sometimes natural conditions significantly slowed decay. Sometimes, the corpse was just replaced again and again with a newly deceased one.
The article above concludes with:
In the face of science, the roman Catholic Church has now virtually abandoned the notion of incorruptibility. It no longer accepts physical preservation as one of the two miracles required before a saint can be recognized by the Pope. Still, suppressing a sense of astonishment is difficult in the presence of a saint's preserved body. It is an amazing affirmation, a testimony to one person's significance in a universe often stony with indifference. It seems to hold out hope that death will not be the end of us, that there is some salvation from the final annihilation that we fear awaits us all.
Since we know that both natural and artificial mummification exist, so the burden of proof lies with anyone who claims that it was the work of god.