Sounds like a big blowtorch or small jet engine when it lights. Love that sound. When you turn that switch to "ON" the switch should glow and a small dot of a light should also come on. When the heater lights successfully, the little dot will go OUT and the light in the switch will say ON. At least the Atwood water heaters I've seen... Some of the older ones DO have a 120-VAC heating element that's hidden on the back (inboard) side of the tank. If you find another mystery switch (ours is a household wall switch with a light that says you've turned it ON), then you may have electric heating also. We can look that up, if you find and post the Model Number.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB