Many (most) Suburban and Atwood water heaters are capable of using both gas and / or electric. On the Suburban water heaters, the on/off switch for the electric is actually on the backside of the water heater buried behind the burner pipe which is accessable by removing the outside water heater cover and searching for it. It is a small white rocker switch that may have a clevis pin through it. Part of the reason they make it somewhat obtuse to use is that if you ever accidentally turn it on with the water heater drained, it is mere seconds before the heating element burns itself up and then you'll have to buy a new element. Most folks don't even know if they have an electric element...the PDI usually doesn't mention it and with no switch on the inside, the element remains an "unknown". If in doubt, go outside and pull the cover off and poke around; it really is hidden, but, you'll probably find it. Some folks even go so far as to wire a switch to control the line from inside the trailer to make it easier, but, the odds of burning up the element rises when you do that.
That being, said, it is possible, if using shore power, to turn it on and let the electric element heat your water without using gas at all. It isn't near as fast as gas, but, is usable and a lot of folks actually use gas and electric simultaneously to heat the water even faster. Gas, overall, is faster, more convenient and less prone to damage and the cost is really small for the amount of gas the heater uses.
I'm in the camp of "if I don't need it, I won't waste it", so, I turn my gas on about 10-15 minutes before I intend to use any really hot water. I turn it on in the morning and turn it off after the 15 minute heat-up and the water stays fairly hot right up until night time when we get ready to do the dishes; then it is on again for about 15 minutes and off for the night and I've never used the electric element on any of the RVs I've had.
Addendum: Even if using gas, there is some electricity involved...just like your refrigerator, it won't work without a 12v source for ignition and thermostat control...you need to have your battery installed; some convertors will still provide 12v without a battery installed, but, some won't (they use the battery to "filter" the convertor's 12v output).
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)