If it’s a rotten egg smell outside near the cover, you may have a loose gas fitting. If the water at the tap smells like that, you may have bacteria growing in the water tank due to a depleted anode rod (unless you have an Atwood heater). One other remote possibility if the odor is only outside is the proximity of the sewer hose and the connections, especially when dumping. More info and description of the odor will help.
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