The yeast and alcohol you have in your beer will keep most of the bad stuff from infecting your beer. You do not need sulfur for this. Just clean and sanitize your keg as you would with bottles, and you will be fine, even at room temperature. A keg is just a big bottle.
Campden tablets usually have a different purpose, for example:
* when making cider, it is used to kill the natural yeast of apple juice before fermenting it so that the yeast you introduce will take over.
* it is also used to stop the fermentation before it ends, for example to keep a cider sweet instead of letting the yeast dry it out.