Your water heater certainly won't "wear out" just because you leave it on all the time, whether gas or electric or both but the biggest downside to running it on electric is the load could under some circumstances pull a breaker, in the trailer itself, at the campsite power post, or even the campground distribution box. Easy to do - A/C is running pulling perhaps 14 amps, you've got the water heater running on electric pulling say 10 amps but you "forget" and decide to run the microwave oven, suddenly increasing your total draw well beyond the 30 amp rating of your campsite. Personally, I try to remember to turn the water heater off during the day when we're just not using hot water but it's likewise easy to forget to turn back on again. Not a big deal if you have to wait 20 mins or so for water hot enough to wash dishes but ask yourself this - are you willing to risk your wife jumping into the shower only to be greeted with cold water because YOU earlier forgot to turn the water heater on?! :E If you value your life there's something to be said for just leaving the water heater on all the time while you're camping. :W
