I'll bet that it's the well water you're filling up with.
When we moved into our present house we almost couldn't shower because of the strong rotten egg smell of the water. Talked with the neighbors and everyone with a well of our depth had the same sulfur smell, neighbors that had drilled significantly deeper didn't. Talked with some water people and it turns out our water has a lot of iron. We installed a small water treatment plant in our basement and now have great water.
Can't really fit the water treatment stuff into a camper so I'd recommend filling up from a different source. You could run water supply for a minute first and smell to make sure you won't have the same problem. Another solution would be to find a way to aerate your water as you fill (will release the gas so what goes into tank won't smell as much).
Good luck